[RFC] Cleanup user documentation 2

follow up to #1011.

  * remove documentation for legacy operating systems
  * remove documentation of features not included in Neovim
  * remove documentation for `:shell` (see #450)
  * fix some links
  * apply changes suggested by @justinmk in #1011 
  * remove links to non existent files

Although I agree we should remove all the related `:shell` documentation, we should leave this one instance here for `:helptags` and future reference, so that users get a useful explanation when they do `:h :shell`. Something like this:

```
							*:sh* *:shell* *E371* *E360*
:sh[ell]		Removed. {Nvim}
```

Notice also the error numbers (E371, E360) are preserved for reference, at least until we determine certainly that we don't need them.

Also, 

- these entries may also be useful to us in the future as a living record of features that were changed/removed in Neovim compared to Vim.
- if we ever restore this feature, we know where to fill it in