docs: Update .readthedocs.yaml as required by readthedocs

readthedocs is now requiring version 2 of .readthedocs.yaml with some fields made mandatory, in order to guarrantee correct environment dependency management. Without this, readthedocs is failing to build the docs as of 25th September 2023.

Link: https://blog.readthedocs.com/migrate-configuration-v2/ Signed-off-by: Ryan Roberts ryan.roberts@arm.com