feat(starters): Add purgeCSS to gatsby-tailwindcss-sass-starter featu…

…res (gatsbyjs#10853)

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## Description
Add PurgeCSS to features
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## Related Issues
N/A
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  Link to the issue that is fixed by this PR (if there is one)
  e.g. Fixes #1234, Addresses #1234, Related to #1234, etc.
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docs/starters.yml

@@ -1558,3 +1558,4 @@
     - SASS/SCSS
     - Comes with React Helmet for adding site meta tags
     - Includes plugins for offline support out of the box
+    - PurgeCSS to shave off unused styles