docs: use the `twitter` Jekyll variable everywhere (#27990)

site/_includes/footer.html

@@ -2,7 +2,7 @@
   <div class="container-fluid p-3 p-md-5">
     <ul class="bd-footer-links">
       <li><a href="{{ site.repo }}">GitHub</a></li>
-      <li><a href="https://twitter.com/getbootstrap">Twitter</a></li>
+      <li><a href="https://twitter.com/{{ site.twitter }}">Twitter</a></li>
       <li><a href="{{ site.baseurl }}/docs/{{ site.docs_version }}/examples/">Examples</a></li>
       <li><a href="{{ site.baseurl }}/docs/{{ site.docs_version }}/about/overview/">About</a></li>
     </ul>

site/docs/4.2/getting-started/introduction.md

@@ -136,11 +136,11 @@ For improved cross-browser rendering, we use [Reboot]({{ site.baseurl }}/docs/{{
 
 Stay up to date on the development of Bootstrap and reach out to the community with these helpful resources.
 
-- Follow [@getbootstrap on Twitter](https://twitter.com/getbootstrap).
+- Follow [@getbootstrap on Twitter](https://twitter.com/{{ site.twitter }}).
 - Read and subscribe to [The Official Bootstrap Blog]({{ site.blog }}/).
 - Join [the official Slack room]({{ site.slack }}/).
 - Chat with fellow Bootstrappers in IRC. On the `irc.freenode.net` server, in the `##bootstrap` channel.
 - Implementation help may be found at Stack Overflow (tagged [`bootstrap-4`](https://stackoverflow.com/questions/tagged/bootstrap-4)).
 - Developers should use the keyword `bootstrap` on packages which modify or add to the functionality of Bootstrap when distributing through [npm](https://www.npmjs.com/search?q=keywords:bootstrap) or similar delivery mechanisms for maximum discoverability.
 
-You can also follow [@getbootstrap on Twitter](https://twitter.com/getbootstrap) for the latest gossip and awesome music videos.
+You can also follow [@getbootstrap on Twitter](https://twitter.com/{{ site.twitter }}) for the latest gossip and awesome music videos.