Fix docs for module-level API

[bdice]

Description

This PR's primary intention is to fix the rmm module API documentation so that classes like DeviceBuffer appear, and to remove documentation for the rmm.rmm submodule in favor of the top-level API that defines all public names. I fixed all warnings that appear when building with Sphinx, and addressed a few other minor issues that didn't deserve their own PRs but were important to fix. I rendered the docs locally and the difference is very substantial -- parameter docstrings, public properties of classes, and other names now automatically appear or render correctly where they weren't before.

Checklist

• I am familiar with the Contributing Guidelines.
• New or existing tests cover these changes.
• The documentation is up to date with these changes.

[bdice]

FYI: this extension was causing havoc with the docs. A lot of things weren't rendering properly, and I would guess it was because of napoleon interacting poorly with Cython-generated docs or a conflict with some other Sphinx extension. The numpydoc extension is sufficient to use on its own nowadays, and napoleon isn't needed here.

[msadang]

Comment and sphinx edits look good.

[bdice]

@gpucibot merge