Paying down some tech debt on docs, runtime API, and cython

[cjnolet]

This PR pays down some tech debt and cleans up some things. On the surface, you'll notice many files have been touched or modified but the modifications are largely confined to a few major categories of changes:

1. Fixes some of the issues found with the doc updates in 22.12.

2. Breaks some of the docs for the c++ namespaces down into multiple sections to make them easier to navigate and consume

3. Renames raft_distance directory into more appropriately named raft_runtime. (This is also in preparation to eventually rename the libraft-distance library into libraft once we can remove the FAISS dependency.

4. Separates out some runtime source files and APIs that were being mistakenly combined with the template specializations API

5. Consolidates multiple mdspan.pxd files into a single file.

6. Consistently uses cpp directory for new(er) pxd files, nested into their respective packages.

7. Consistently uses doxygen groups in many of the namespaces.

[benfred]

this looks great! I'm liking how much more fleshed out the docs will be here -

[cjnolet]

@gpucibot merge