fix(docs): add readthedocs configuration file

Readthedocs uses weird defaults and the web interface gives limited
configuration options. Add the config file to allow them to be changed.

Bump build os image to Ubuntu 22.04 to be in line with the CI.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: I1a620b15ab3924244f305056096024fe117c63dd