[doc] Add Discord invite link alongside channel links (#126352)

By far the most important part of this patch is updating
GettingInvolved.rst to include the invite link, but I've grepped for any
other discord.com links.

I'm no Discord expert, but from my experience (confirmed via @preames
kindly testing as well) the direct channel links provide a confusing
experience if you haven't already found and used an invite link to the
LLVM Discord server. If you're logged into Discord but not a member of
LLVM's sever, the web app opens and then...nothing. No channel opens, no
prompt to join the server or even a hint that you need to find an invite
link (and if you're not used to Discord, you likely don't even know
that's necessary).

This patch addresses the issue by providing the invite link where
Discord is mentioned.

clang/www/OpenProjects.html

@@ -150,7 +150,7 @@ a text editor could change.</p></li>
 that demonstrates the problem down to something small. There are many ways to
 do this; ask on <a href="https://discourse.llvm.org/c/clang">Discourse</a>,
 <a href="https://discord.com/channels/636084430946959380/636725486533345280">Discord</a>
-for advice.</p>
+(<a href="https://discord.gg/xS7Z362">invite link</a>) for advice.</p>
 
 </div>
 </body>

libc/docs/index.rst

@@ -12,7 +12,8 @@ The LLVM C Library
   LLVM-libc is not fully complete right now. Some programs may fail to build due
   to missing functions. If you would like to help us finish LLVM-libc, check
   out "`Contributing to the libc project <contributing.html>`__" in the sidebar
-  or ask on `discord <https://discord.com/channels/636084430946959380/636732994891284500>`__.
+  or ask on `discord <https://discord.com/channels/636084430946959380/636732994891284500>`__
+  (`invite link <https://discord.gg/xS7Z362>`__).
 
 Introduction
 ============

libcxx/docs/index.rst

@@ -242,5 +242,5 @@ Quick Links
 * `LLVM Bug Tracker <https://github.com/llvm/llvm-project/labels/libc++/>`_
 * `libcxx-commits Mailing List <http://lists.llvm.org/mailman/listinfo/libcxx-commits>`_
 * `libc++ forum <https://discourse.llvm.org/c/runtimes/libcxx/>`_
-* `libc++ chat <https://discord.com/channels/636084430946959380/636732894974312448>`_
+* `libc++ chat <https://discord.com/channels/636084430946959380/636732894974312448>`_ (`invite link <https://discord.gg/xS7Z362>`_)
 * `Browse libc++ Sources <https://github.com/llvm/llvm-project/tree/main/libcxx/>`_

lldb/docs/index.rst

@@ -182,7 +182,8 @@ interesting areas to contribute to lldb.
 
    Source Code <https://github.com/llvm/llvm-project>
    Releases <https://github.com/llvm/llvm-project/releases>
-   Discord <https://discord.com/channels/636084430946959380/636732809708306432>
+   Join the Discord <https://discord.gg/xS7Z362>
+   Discord Channel <https://discord.com/channels/636084430946959380/636732809708306432>
    Discussion Forums <https://discourse.llvm.org/c/subprojects/lldb/8>
    Developer Policy <https://llvm.org/docs/DeveloperPolicy.html>
    Bug Reports <https://github.com/llvm/llvm-project/issues?q=is%3Aissue+label%3Alldb+is%3Aopen>

llvm/docs/GettingInvolved.rst

@@ -392,7 +392,7 @@ Discord
 -------
 
 Users and developers of the LLVM project (including subprojects such as Clang)
-can be found on the community's `Discord <https://discord.com/channels/636084430946959380/636725486533345280>`_
+can be found on the community's `Discord <https://discord.gg/xS7Z362>`_
 chat server. The server is actively moderated.
 
 The #buildbot-status channel has a bot for