[FEA] Improve doxygen documentation

[vyasr]

Is your feature request related to a problem? Please describe.
In the process of working through issues in #1324 we identified a number of improvements that we'd like to see in rmm documentation. This issue attempts to capture those items for future improvements.

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1. Add documentation for each doxygen group
2. Add documentation of the rmm namespace
3. Use a better theme for the documentation to improve organization and reduce how cramped it is
4. Improve the ToC and the appearance in the sidebar
5. Manually curate the order of items in each group because the default doxygen order puts distracting things at the top