Docs core pages spacing and typography updates (#118)

* Global padding to 12px, 32px margin-top to h3 and h4, h3 to 18px and face to Sora, h4 to 16px, all paragraph text color to text-secondary

* Add client reference test content

* Increase header sizes, increase margin-top on headers, change h4 color to tertiary, reset global margin to 16px

* Correct h4 color var

* Move h4 color var to correct line

* Increase h sizes, increase h3, 4 margin-top

* Update client reference examples

* Update preview source nav

* Increase p line height, decrease inline code size, decrease list spacing

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Co-authored-by: Eric Schneider <[email protected]>