Update cassini branch automatically

In the documentation, the cassini branch is set from:

layer_definitions["meta-cassini branch"]

This requires a manual update for each new stable branch and is currently wrong on main. To avoid having to update this variable for each new stable branch, set the value from the environment.

On Read The Docs READTHEDOCS_GIT_IDENTIFIER can be used.

In CI we replicate this behaviou by setting READTHEDOCS_GIT_IDENTIFIER from CI_COMMIT_REF_NAME

You can see the read the docs output here

You can see the CI output here