Cherrypick Bzlmod documentation (bazelbuild#14301)

* Bzlmod: Add documentation

(bazelbuild#13316)

Adding first draft of Bzlmod documentation. The part about module extensions is still WIP and will be added later this week. The entire page is expected to be cherrypicked into the 5.0 release.

PiperOrigin-RevId: 410606833

* Bzlmod: More documentation about repo names and strict deps

RELNOTES:
PiperOrigin-RevId: 410615340

* Bzlmod: Generate stardoc for module_extension() and tag_class()

(bazelbuild#13316)

Also fixes some minor issues with existing docstrings (dangling links, etc)

PiperOrigin-RevId: 410880750

* Bzlmod: Documentation on module extensions

(bazelbuild#13316)

PiperOrigin-RevId: 411038059

Co-authored-by: pcloudy <pcloudy@google.com>

site/_includes/getting-started-sidebar/master/sidebar.html

@@ -36,6 +36,7 @@ <h3>Understanding</h3>
 <ul class="sidebar-nav">
   <li><a href="/versions/{{ current_version }}/build-ref.html">Core Concepts</a></li>
   <li><a href="/versions/{{ current_version }}/external.html">External Dependencies</a></li>
+  <li><a href="/versions/{{ current_version }}/bzlmod.html">Bzlmod User Guide</a></li>
   <li><a href="/versions/{{ current_version }}/configurable-attributes.html">Configurable Attributes</a></li>
   <li><a href="/versions/{{ current_version }}/platforms-intro.html">Platforms and Toolchains</a></li>
   <li><a href="/versions/{{ current_version }}/visibility.html">Visibility</a></li>