Deprecation of Sphinx context injection

Read the Docs will stop defining `html_baseurl` Sphinx configuration,
which means that projects will need to define it by themselves to keep the
canonical custom domain properly configured.

The `READTHEDOCS_CANONICAL_URL` environment variable is used to define
`html_baseurl` to keep the previous behavior.

Also inject the `READTHEDOCS` variable into the `html_context`.

Code fragment taken from the blog post here:
https://about.readthedocs.com/blog/2024/07/addons-by-default/



Signed-off-by: Hugues KAMBA MPIANA <hugues.kambampiana@arm.com>