MOTECH-899: Set up Sphinx and ReadTheDocs.org

For platform-campaigns repo, renamed HomePage.txt to index.txt so that
ReadTheDocs.org can find the top-level page. Edited conf.py to reflect
this renaming.

Change-Id: Ic9a0fc122f46f176d813cf25a2af268970564fa5
Merge "MOTECH-899: Set up Sphinx and ReadTheDocs.org"