tc: add support for the .readthedocs.yaml RTD config file

Add support for the .readthedocs.yaml file which specifies the
RTD docs project config, mandatory for the current rendering
of the TC documentation for public releases.
This file already includes recent required build.os config key
imposed by the Oct 16, 2023 public RTD support changes.

Signed-off-by: Sergio Alves <sergio.dasilvalves@arm.com>