Fix most sphinx warnings

[JeanChristopheMorinPerso]

This PR fixes most Sphinx warnings. This this, I was able to remove 115 warnings. There are only two warnings left:

<unknown>:1: WARNING: more than one target found for cross-reference 'Command': rez.command.Command, rez.rex.Command [ref.python]
<unknown>:1: WARNING: py:class reference target not found: ModuleType [ref.class]

These two warnings are bugs in Sphinx. The autodoc extension was recently rewritten and it seems to have a few bugs. The warnings don't affect the builds though, so it's all good.

AI disclosure: This started out with an LLM, but I ended up abandoning all its suggestion and instead I did all the changes manually, all by myself. So everything in this PR is coming from my brain.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.29%. Comparing base (5c598c5) to head (20c0770).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2182   +/-   ##
=======================================
  Coverage   61.29%   61.29%           
=======================================
  Files         164      164           
  Lines       20568    20568           
  Branches     3575     3575           
=======================================
  Hits        12607    12607           
  Misses       7089     7089           
  Partials      872      872           

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[Copilot]

Pull request overview

Improves Sphinx API documentation and suppresses warnings from inherited and unresolved types.

Changes:

• Corrects docstring markup and cross-references.
• Expands generated API documentation.
• Filters noisy inherited-member warnings and enables parallel builds.

Reviewed changes

Copilot reviewed 8 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
.gitignore	Ignores generated vendor API pages.
docs/Makefile	Enables parallel builds and cleans vendor pages.
docs/source/api.rst	Adds plugin and vendored API entries.
docs/source/conf.py	Configures warning ignores and member filtering.
src/rez/package_repository.py	Documents the repository manager singleton.
src/rez/rex.py	Documents and types environment keys.
src/rez/util.py	Clarifies a return-value docstring.
src/rez/utils/pip.py	Corrects docstring type references.
src/rezplugins/package_repository/memory.py	Improves repository documentation markup.

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[sanikache]

Confirmed I now only see the two warnings you mentioned.

copying static files: done
copying extra files... 
copying extra files: done
copying assets: done
<unknown>:1: WARNING: more than one target found for cross-reference 'Command': rez.command.Command, rez.rex.Command [ref.python]
<unknown>:1: WARNING: py:class reference target not found: ModuleType [ref.class]
writing output... [100%] variants
generating indices... genindex py-modindex done
writing additional pages... search done
copying images... [100%] _static/pkg_path_anatomy.png
dumping search index in English (code: en)... done
dumping object inventory... done
build succeeded, 2 warnings.

The HTML pages are in _build/html.

[sanikache]

LGTM.