Docs: Enable move to RTD

Add .reathedocs.yaml file which is used to build the documentation.
Edit docs/conf.py to avoid KeyError on RTD builds.

Change-Id: Idbe7f98e5333a82b92dbedd5a43696fe55fd1ed4
Signed-off-by: Elena Uziunaite <elena.uziunaite@arm.com>
2 files changed