Document plugin settings

[BryceGattis]

As a rez user, I'd like plugin settings to be documented in our documentation so I can easily change their settings.

Related Issues/PRs

• Convert all plugin rezconfig to rezconfig.py #1692 (normalizes all plugin rezconfig files to Python)
• Add releasing packages documentation #1689 (comment) (JC describing the issue)

Note that we now have a proper documentation for plugins (https://rez.readthedocs.io/en/stable/plugins.html) and it'll be improved even more with #1872.

Requirements:

• Must be automatically generated like the rest of the settings in https://rez.readthedocs.io/en/stable/configuring_rez.html#configuration-settings.
• Must either live in https://rez.readthedocs.io/en/stable/configuring_rez.html#configuration-settings or https://rez.readthedocs.io/en/stable/plugins.html. To be discussed.