doc: add read the docs config file

Use the configuration file to specify the host OS and python version
to render the docs.

Currently, only Ubuntu 20.04 is supported.

Signed-off-by: Nathan Brown <nathan.brown@arm.com>
Reviewed-by: Dhruv Tripathi <dhruv.tripathi@arm.com>

Change-Id: I60f779aded7aeb215c3bc2a496c21f5980fc69cd