[RFC] 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:

- [x] add `vim-patch:0` to commit message
- [x] credit contributors of [docs](https://github.com/neovim/docs) and [vimscript](https://github.com/neovim/vimscript) repos in the commit message
- [x] move issues from [docs](https://github.com/neovim/docs) to main Neovim repo
- [x] move issues from [vimscript](https://github.com/neovim/vimscript) to main Neovim repo
- [x] remove `tutor/`, `spell/` and some other directories for the time being, until it is clear that we want to maintain these (it may also make more sense for these to be downloaded as plugins instead of of being distributed with the Neovim package). *These can always be added later if needed!*
- [x] verify/repair/create Makefile/CMake targets (note @rjw57 work in https://github.com/neovim/docs/commits/master)


More details/background on why we're moving away from the separate repos: https://github.com/neovim/neovim/pull/633#issuecomment-47355315

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**Update:**

The last the two tasks above are not complete, but I committed `runtime/` in order to unblock several PRs.