[WIP] Initial documentation of nvim features

Starting some documentation so users read about Neovim from within nvim!
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:

```
    *msgpack_rpc.txt*    For Nvim version 0.0.1                         {Nvim}
                                                                              ^
                                                                              |
                                                                        [80th column]
```