Make complex setting values like latex_documents overridable

Is your feature request related to a problem? Please describe.
I want to override latex specific settings when invoking sphinx-build to embed the generated latex in an external document. I do not want to edit the original conf.py. First I tried a dedidcated conf.py but ran into problems with relative paths so I decided to try the command line settings override.

The --define documentation nicely explains how to override dict and list type values. However, latex_documents is a list of tuples which are not handled correctly.

Describe the solution you'd like
I do not have a favourite solution.

My original problem of patching "conf.py" without file modification can also be solved by additional "appendable" python code.

Describe alternatives you've considered

• Make list entries addressable by <name>.<index> notation
• Intoduce dedicated syntax for latex_document entry that does not use ,
• Add a dedicated --json-settings argument which accepts a JSON object. This can be tricky to use because shells treat quotes differently.

Additional context
Parsing of list type values (btw: There is no way to escape the ","): https://github.com/sphinx-doc/sphinx/blob/6471027d1936f0a0a38ef281862f0b29c53618f6/sphinx/config.py#L342

Interpretation of list elements as sequence in the latex builder: https://github.com/sphinx-doc/sphinx/blob/6471027d1936f0a0a38ef281862f0b29c53618f6/sphinx/builders/latex/__init__.py#L150

Start with the --define documentation and the list-value parsing in sphinx/config.py around line 342, then inspect how the LaTeX builder interprets sequence values around sphinx/builders/latex/init.py line 150. Compare the proposed override approaches and establish a supported way to override latex_documents without editing conf.py; done means complex values are parsed and applied correctly.