• This introduces a breaking change for users overriding the new (undocumented) jobs (create_environment, install, build.html and friends). This is, if they have overridden any of those jobs without explicitly declaring a Sphinx or MkDocs key, we will no longer run the sphinx/mkdocs commands, not their setup.
• This hardcodes the dates from Post: deprecate config files without sphinx or mkdocs configuration website#342 to through an error to users using the configuration file without an explicit sphinx/mkdocs configuration.

This allows for users to keep using the new overrides without worrying about sphinx/mkdocs, while giving enough time to old users to migrate their projects to give an explicit path.

Some notes

• We are allowing to use sphinx/mkdocs and probably other keys with build.commands, even if those keys don't affect the build in anything. Not really something that "interrupt" users in any way, but it can be missleading.
• A next step should be to not make python required at all, right now we still create a virtual environment. We probably want to create a virtual env only if python was provided in the list of build.tools.

Ref:

• Post: deprecate config files without sphinx or mkdocs configuration website#342
• Deprecation: stop auto-finding conf.py/mkdocs.yml from the repository #10637
• Config: avoid default sphinx/mkdocs steps #11819