split required characters for custom font into its own section

docs/modules/theme/pages/prepare-custom-font.adoc

@@ -40,26 +40,56 @@ Resaving the font in FontForge will usually resolve any errors in the font.
 To ready your font for use with Asciidoctor PDF, you'll need to modify it using a font program.
 We recommend using {url-fontforge}[FontForge^].
 But don't let this scare you off.
-FontForge essentially works like a vector-drawing tool, in which each character is a separate canvas.
+FontForge essentially works like a vector-drawing tool, in which each character is a separate glyph canvas.
 You can find a crash course in how to use the program on the FontForge project site.
 
 Here are the modifications you need to apply to a custom font for it to work best with Asciidoctor PDF:
 
 * Convert the font to TTF or OTF (only required if the font is a TTC or other format not supported by Prawn).
-* Add the glyphs for the required characters if missing from the font (optional if using a fallback font).
+* Add the glyphs for the <<required-characters,required characters>> if missing from the font (optional if using a fallback font).
 * Subset the font to exclude unused characters to reduce the file size (optional).
 * Save the file using the old-style kern table to activate kerning.
 
-NOTE: Technically, subsetting the font (i.e., removing glyphs) is not required since Prawn only embeds the characters from the font used in the document (i.e., it automatically subsets the font).
-However, if you plan to commit the font to a repository, subsetting helps keep the file size down.
+Subsetting the font (i.e., removing glyphs) is not required since Prawn only embeds the glyphs from the font used in the document (i.e., it automatically subsets the font).
+However, if you plan to commit the font to a repository, subsetting helps keep the file size manageable.
 
 Most fonts do not provide glyphs for all the Unicode character ranges (i.e., scripts).
 (A glyph is the corresponding vector image for a Unicode character).
 In fact, many fonts only include glyphs for Latin (Basic, Supplement, and Extended) and a few other scripts (e.g., Cyrillic, Greek).
 That means certain glyphs Asciidoctor PDF relies on may be missing from the font.
+The <<Required characters>> section lists all the characters that a custom font is expected to provide.
+
+One way to get the glyphs for these required characters is to steal them from another font (or from another character in the same font).
+To do so, open the other font in FontForge, select the character, press kbd:[Ctrl,c], switch back to your font, select the character again, and press kbd:[Ctrl,v].
+You may need to scale the glyph so it fits properly in the art box.
+
+IMPORTANT: If you're copying a non-visible character, be sure to set the width to 0 using menu:Metrics[Set Width...], enter 0 into *Set Width To*, then click btn:[OK].
+
+When you're adding any required characters, save the font with the old-style kern table enabled.
+To do so, select menu:File[Generate Fonts...], select *TrueType*, click btn:[Options], make sure _only_ the following options are checked (equivalent to the flags 0x90 + 0x08):
+
+* [x] OpenType
+** [x] Old style 'kern'
+
+Then click btn:[OK], then uncheck *Validate Before Saving*, and finally click btn:[Generate] to generate and save the font.
+
+Your font file is now ready to be used with Asciidoctor PDF.
+
+[#scripting-modifications]
+== Scripting the font modifications
+
+Performing all this font modification manually can be tedious (not to mention hard to reproduce).
+Fortunately, FontForge provides a {url-fontforge-scripting}[scripting interface^], which you can use to automate the process.
+
+In fact, that's what we use to prepare the fonts that are bundled with Asciidoctor PDF.
+You can find that FontForge script, the Bash script that calls it, and the Docker image in which it is run in the {url-project-repo}/tree/{page-origin-refname}/scripts[scripts directory^] of this project.
+You can use that script as a starting point or reference for your own font preparation / modification script.
+
+[#required-characters]
+== Required characters
 
 Below are the non-Latin characters (identified by Unicode code point) on which Asciidoctor PDF relies that are often missing from fonts.
-Unless you're using a fallback font that fills in the missing glyphs, you need to ensure these glyphs are present in your font (and add them if not).
+You need to ensure these glyphs are present in your prepared font or configure a fallback font that provides them.
 
 * \u00a0 - no-break space
 * \ufeff - zero width no-break space
@@ -78,36 +108,14 @@ Unless you're using a fallback font that fills in the missing glyphs, you need t
 * \u25ba - right pointer (used for media play icon when icon fonts are disabled)
 * .notdef - used as the default glyph when a requested character is missing from the font (usually a box)
 
-NOTE: If the .notdef glyph is non-empty (i.e., contains splines), it will be used as the default glyph when the document requests a character that is missing from the font.
-Unlike other glyphs, the .notdef glyph is referenced by name only.
-It does not have a designated Unicode code point.
+If the .notdef glyph is non-empty (i.e., contains splines), it will be used as the default glyph when the document requests a character that's missing from the font.
+Unlike other glyphs, the .notdef glyph is referenced by name only, meaning it does not have a designated Unicode code point.
 
 If you're preparing a font for use in verbatim blocks (e.g., a listing block), you'll also need this range of characters:
 
 * \u2460 to \u2468 - circled numbers
 
-One way to get these glyphs is to steal them from another font (or from another character in the same font).
-To do so, open the other font in FontForge, select the character, press kbd:[Ctrl,c], switch back to your font, select the character again, and press kbd:[Ctrl,v].
-You may need to scale the glyph so it fits properly in the art box.
-
-IMPORTANT: If you're copying a non-visible character, be sure to set the width to 0 using menu:Metrics[Set Width...], enter 0 into *Set Width To*, then click btn:[OK].
-
-When you're done, save the font with the old-style kern table enabled.
-To do so, select menu:File[Generate Fonts...], select *TrueType*, click btn:[Options], make sure _only_ the following options are checked (equivalent to the flags 0x90 + 0x08):
-
-* [x] OpenType
-** [x] Old style 'kern'
-
-Then click btn:[OK], then uncheck *Validate Before Saving*, and finally click btn:[Generate] to generate and save the font.
+The fonts used by the default theme in Asciidoctor PDF provide all of these glyphs.
 
-Your font file is now ready to be used with Asciidoctor PDF.
-
-[#scripting-modifications]
-== Scripting the font modifications
-
-Performing all this font modification manually can be tedious (not to mention hard to reproduce).
-Fortunately, FontForge provides a {url-fontforge-scripting}[scripting interface^], which you can use to automate the process.
-
-In fact, that's what we use to prepare the fonts that are bundled with Asciidoctor PDF.
-You can find that FontForge script, the Bash script that calls it, and the Docker image in which it is run in the {url-project-repo}/tree/{page-origin-refname}/scripts[scripts directory^] of this project.
-You can use that script as a starting point or reference for your own font preparation / modification script.
+If you don't want to add additional glyphs to the font you are preparing, you can configure a fallback font instead.
+A symbol font, such as Symbola, is a good choice as a fallback font since it's focus is on filling in these glyphs for other fonts.