Add ability to create a section of help from multiple repositories

[rmglennon]

Currently, our docs pull together multiple repositories into one help system. It would be handy to expand this to allow an individual section of help (one config file) to pull from multiple repositories.

This came up recently with Android. We'd like the ability for the Android docs to pull from two source repos, but could not come up with a clean way to do this in our current build process because each config has only one base URL and GitHub source.

I found an issue in the mk-docs repository about this, but it was closed and deemed a relatively esoteric workflow...