Create submodules for 4.0, 4.1 and master docs

I was having a look locally and added the docs as submodules because it was just as easy as cloning the relevant branch. Then I figured, it might be easier for people editing the docs locally to use submodules as they'd be working in the correct repository/branch and still see their changes as they're shown on the site.

However this may be completely unhelpful depending on your existing deployment setup :smile: What do you think?