Fix sphinx warnings

[JeanChristopheMorinPerso]

Fix Sphinx warnings caused by duplicate symbols by introducing two new Sphinx domains: rex and pkgdef.

This makes it possible to reference package attributes in the docs using :pkgdef:attr for example, and stuff in commands using :rex:attr.

This removes any duplicates in symbols. These new domains won't affect users reading our docs in any way, but it will have an impact on any user that are cross-linking our docs using intersphinx (https://docs.readthedocs.com/platform/latest/guides/intersphinx.html). These users will have to update references to :data:version, etc to use our domains instead.

I also fixed some more warnings that were not related to the duplicate symbols, and modified the CLI docs auto-generation code to improve the readability of the docs a little bit (please look at https://rez--2062.org.readthedocs.build/en/2062/commands_index.html?readthedocs-diff=true to visualize the diff).

AI disclosure:

The custom domains code was written using Amp. Here are the threads:

• https://ampcode.com/threads/T-019b854a-b579-770f-bbd2-6f3c7283d9e1
• https://ampcode.com/threads/T-019b855b-4386-74db-a9c1-163b3a62f297
• https://ampcode.com/threads/T-019b8568-e761-736f-b079-246e9724cf42
• https://ampcode.com/threads/T-019b857b-ab87-73df-acf4-1faae6673a88

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 60.00%. Comparing base (a0d2ddb) to head (a2e6053).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2062   +/-   ##
=======================================
  Coverage   59.99%   60.00%           
=======================================
  Files         163      163           
  Lines       20118    20118           
  Branches     3519     3519           
=======================================
+ Hits        12070    12071    +1     
+ Misses       7227     7226    -1     
  Partials      821      821           

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