doc: Use READTHEDOCS_GIT_IDENTIFIER in conf.py

Recent versions of Read the Docs set the environment variable
READTHEDOCS_GIT_IDENTIFIER to the value of the checked out version
(e.g. branch or tag name). Use this instead of the custom
RTD_ENV_KRONOS_VERSION.

Additionally check the variable CI_COMMIT_REF_NAME so that
documentation built by Gitlab has the correct version name.

Signed-off-by: Peter Hoyes <peter.hoyes@arm.com>