[WIP] re-integrate runtime/

re-integrating these separate repos into the core repo under the `runtime/` folder:

- https://github.com/neovim/vimscript
- https://github.com/neovim/docs

I resisted the urge to rename `runtime` to something like `user` or `share`, because considering the contents of the directory it is really an appropriate name. I looked at projects like emacs and git for alternative precedents, and came away with the impression that Vim is actually a pretty nicely organized project: user-deployable files are in a single `runtime/` folder instead of scattered and glued together by a build script.

todo:

- [ ] remove some possibly-irrelevant such as tools/ as was done in the [vimscript repo](https://github.com/neovim/vimscript/commits/master)
- [ ] ...
- [ ] verify/repair/create Makefile/CMake targets (also note @rjw57 work in https://github.com/neovim/docs/commits/master )
- [ ] squash