Add support for versionremoved directive

Note that I added a snippet in the Kitchen Sink to preview the change. I have an open PR in sphinx-themes/sphinx-themes.org#163 to add it upstream. I can remove this snippet, if you prefer, or you can clobber it the next time you run a sync.

I'll also attach a screenshot.

- Closes pydata#2086

docs/examples/kitchen-sink/admonitions.rst

@@ -121,3 +121,10 @@ Sphinx provides several different types of admonitions.
 .. deprecated:: v0.1.1
 
    Here's a deprecation message.
+
+``versionremoved``
+==================
+
+.. versionremoved:: v0.1.1
+
+   Here's a version removed message.

src/pydata_sphinx_theme/assets/styles/components/_versionmodified.scss

@@ -1,6 +1,7 @@
 div.versionadded,
 div.versionchanged,
-div.deprecated {
+div.deprecated,
+div.versionremoved {
   vertical-align: middle;
   margin: 1.5625em auto;
   padding: 0 0.6rem;
@@ -34,7 +35,8 @@ div.versionchanged {
   background-color: var(--pst-color-warning-bg);
 }
 
-div.deprecated {
+div.deprecated,
+div.versionremoved {
   border-color: var(--pst-color-danger);
   background-color: var(--pst-color-danger-bg);
 }
@@ -64,9 +66,10 @@ span.versionmodified.changed {
   }
 }
 
-span.versionmodified.deprecated {
+span.versionmodified.deprecated,
+span.versionmodified.removed {
   &::before {
     color: var(--pst-color-danger);
-    content: var(--pst-icon-versionmodified-deprecated);
+    content: var(--pst-icon-versionmodified-removed);
   }
 }

src/pydata_sphinx_theme/assets/styles/variables/_versionmodified.scss

@@ -7,4 +7,5 @@ html {
   --pst-icon-versionmodified-added: var(--pst-icon-exclamation-circle);
   --pst-icon-versionmodified-changed: var(--pst-icon-exclamation-circle);
   --pst-icon-versionmodified-deprecated: var(--pst-icon-exclamation-circle);
+  --pst-icon-versionmodified-removed: var(--pst-icon-exclamation-circle);
 }