[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 (Own Type Functions Don't Work Within Namespaces #1101 and A compilation error using the clang compiler on Android. #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 Lua Sol3 and C++ (Oh, My!) #1081)
• Write about iterator invalidation when using C++ containers in Lua (see a user example from Segmentation fault when sol::container::erase #1237)

Lots to do for sol4....