Auto-update the Sphinx API documentation for Rez

[ColinKennedy]

Rez currently has generated API documentation files, here: https://github.com/nerdvegas/rez/tree/master/docs/api

However it's missing many files. It'd be really great if these pages were kept up to date with each Rez release. I've done something similar to this before and here's how I think Rez could handle it.

In or before build.py is ran, we call sphinx-apidoc and give it arguments like:

sphinx-apidoc src --output-dir docs/api

Or if called within build.py, it would look like this:

from sphinx.ext import apidoc

def _cli(...):
    # ... more code ...
    apidoc.main([os.path.join(REZ_SOURCE_DIR, "src"), "--output-dir", os.path.join(THIS_DIR, "api")])
    # ... more code ...

Then when GitHub builds the docs via https://github.com/nerdvegas/rez/blob/master/.github/workflows/sphinx.yaml#L21, the API files are added just before sphinx-build is ran and everything is kept up to date.

Motivation
In code that uses Rez API, I frequently refer to rez.resolved_context.ResolvedContext, rez.packages.Package, and rez.packages.Variant which are all currently there. However other additions like rez.developer_package.DeveloperPackage and rez.vendor.version.version.Version are missing. DeveloperPackage in particular is used a lot and it's a shame to not have links to it.

[ColinKennedy]

Note: an existing PR implements this feature: #1295

[JeanChristopheMorinPerso]

Fixed in #1522