[v4] Rewrite with Doxygen

[ThePhD]

I'm pretty happy with Sphinx + Doxygen, and will probably move to do that over any other setup. I just need to pump it into sol2 now, which means it's time to aggressively document every public entity, and move everything that's not public into the various __detail namespaces.

There's a lot of work that needs to be done while we do this, too. Including:

• [ ] Rename all args to use the postfix underscore convention, everywhere
• [ ] Promote some of the detail functions out and make them formal
• [ ] Document the extension points better
• [ ] Document ADL mechanism for extension points to avoid confusion (#1101 and #1228) to avoid confusion for specific types and what is considered an associated namespace
• [ ] Write more examples to be embedded in the doxygen comments
• [ ] Make a section for in-the-wild usage, testimonials, and averts. (e.g., articles from #1081)
• [ ] Write about iterator invalidation when using C++ containers in Lua (see a user example from #1237)

Lots to do for sol4....