[WIP] Initial documentation of nvim features

Starting some documentation so users read about Neovim from within nvim!
> we'll probably want to remove these hard-coded dates in the various /runtime/ files sooner or later.

:+1: . Maybe they are remnants from a time vim didn't use a VCS?

> Also, I think we should mark neovim-specific sections with a token like {Nvim} so that the tag system (and the HTML generator) can know which features are Neovim-specific. For files where the entire document is Neovim-specific, perhaps {Nvim} should be in the very first line of the file. Something like this:

:+1: