Explore dynamic base URLs

[alandefreitas]

In PRs and other branches of a project, the links to the source code created by mrdocs with baseURL are wrong for the changes in that commit.

For instance, in https://46.buffers.prtest.cppalliance.org/buffers/reference/boost/buffers/single.html, the URL should be https://github.com/vinniefalco/buffers/blob/develop/include/boost/buffers/buffer.hpp#L559 (or preferably a URL with the commit id), but the base URL in the mrdocs.yml file is https://github.com/cppalliance/buffers/blob/develop/include.

This is more about the tooling around mrdocs than about mrdocs. But I still don’t know what a good solution to that would be.