docs: update theme wording and remove wiki link (#52)

- Replace "built in themes" with "official themes"
- Remove ProperDocs Themes wiki link

Author: DevTwilight

docs/index.md

@@ -28,12 +28,11 @@ configuration file. Start by reading the [introductory tutorial], then check the
         <h3 class="card-title">Great themes available</h3>
         <p class="card-text">
             There's a stack of good looking <a href="user-guide/choosing-your-theme">themes</a> available for ProperDocs.
-            Choose between the built in themes:
+            Choose between the official themes:
             <a href="user-guide/choosing-your-theme/#mkdocs">mkdocs</a> and
             <a href="user-guide/choosing-your-theme/#readthedocs">readthedocs</a>,
             select one of the third-party themes
-            (on the <a href="https://github.com/properdocs/properdocs/wiki/ProperDocs-Themes">ProperDocs Themes</a> wiki page
-            as well as the <a href="https://github.com/properdocs/catalog#-theming">ProperDocs Catalog</a>),
+            (on the <a href="https://github.com/properdocs/catalog#-theming">ProperDocs Catalog</a>),
             or <a href="dev-guide/themes/">build your own</a>.
         </p>
       </div>