Added developer doc on source code repo layout

Based on @paulproteus suggestion on IRC, I've added information on the oh-mainline source code repo layout. I've based it on ``LAYOUT.rst`` found in the root directory of oh-mainline.

Hopefully, this will help new developers more easily understand the openhatch code base.

Also, I included a minor doc formatting change to ``advanced_installation.rst`` so the ``render_docs.py`` tool could build without errors.