[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

But don't worry about doing this for `:wsverb` or netbeans or `:sniff`. It's not worth it.