Add master_doc value index to conf.py

Add master_doc value index to conf.py

Not sure why but read the docs is throwing an error:

Running Sphinx v1.8.5
loading translations [en]... done
making output directory...
building [mo]: targets for 0 po files that are out of date
building [readthedocs]: targets for 5 source files that are out of date
updating environment: 5 added, 0 changed, 0 removed
reading sources... [ 20%] config-deploy
reading sources... [ 40%] developer-guide
reading sources... [ 60%] index
reading sources... [ 80%] overview
reading sources... [100%] release-notes

Sphinx error:
master file
/home/docs/checkouts/readthedocs.org/user_builds\
/o-ran-sc-portal-ric-dashboard-test/checkouts/master/docs/contents.rst
not found

I think this will fix it..
(just a quick google didnt look too hard)

Signed-off-by: Aric Gardner <agardner@linuxfoundation.org>
Change-Id: If56b5da595b47461111a03ef88e7178b85dc1f00