docs: Deprecation of Sphinx context injection

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

Use READTHEDOCS_CANONICAL_URL environment variable to define
html_baseurl to keep the previous behavior.

Also inject 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: Vineeth Raveendran <vineeth.raveendran@arm.com>