Improve configuration environment variable docs

[JeanChristopheMorinPerso]

We've had a gap for years now in regards to config settings environment variables. I partly filled that gap when I introduced https://rez.readthedocs.io/en/stable/environment.html and the automatic documentation of env vars in settings (https://rez.readthedocs.io/en/stable/configuring_rez.html#envvar-REZ_CONTEXT_TMPDIR), but there was still a major missing piece.

Some of our settings can only be configured via an environment variable suffixed with _JSON.

This PR properly documents those and also documents that all settings be set via REZ_XXX_JSON. I also took the occasion to document the format of some env vars too.

With this, I think we can finally say that all our settings and env vars are properly documented. At least I think, until someone comes in to tell me that we missed something 😄

Lastly, I also fixed an old bug that was triggered when a user was setting a setting via an env var but without the JSON suffix for a setting that only supports JSON. Rez was raising a fat from helpful NotImplementedError. With the fix, rez raises an actionable exception.

Amp-Thread-ID: https://ampcode.com/threads/T-01a02b3d-dd38-7533-abe8-4844dadda902

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 61.31%. Comparing base (5c598c5) to head (5fe4492).
⚠️ Report is 7 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2189      +/-   ##
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+ Coverage   61.29%   61.31%   +0.01%     
==========================================
  Files         164      164              
  Lines       20568    20577       +9     
  Branches     3575     3576       +1     
==========================================
+ Hits        12607    12616       +9     
  Misses       7089     7089              
  Partials      872      872              

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[sanikache]

Is it worth adding a new test that guards against any Setting subclass referenced in config_schema that doesn't set env_var_json_only = True ?