Import docs from wiki.

Pull the wiki into a `/doc` folder. This workflow works better for a few reasons:
- People can submit PRs to modify documentation even if they don't have commit access
- Documentation can be updated in the same commit/PR as the change itself

If no-one in @Homebrew/owners has any massive objections to the concept itself I'll make sure all the links work, add the titles and improve linking generally so that it works like the wiki used to.

Just before this is merged I'll clean up the main wiki entry points to point to their respective `/doc` pages.

Thoughts?