Use PyData docs theme, reorganize C++ docs

[bdice]

Description

This is split off of #2087.

I am overhauling the RMM documentation. This is the first set of changes, which includes a new theme and a reorganization of the C++ docs. All docs now use Markdown / Myst.

The next phases will include docstring tweaks to fix various formatting/cross-linking issues (see #2138 and #2139 for current progress on this), an expansion of the Python API docs, and adding user guides for various features.

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]

/merge