Add examples and/or usage description of `autosummary_filename_map`

Is your feature request related to a problem? Please describe.

My problem is essentially described by this comment that I left in the repo of sphinx.automodapi
https://github.com/astropy/sphinx-automodapi/issues/73#issuecomment-1466259565

I am not sure if the issue is fixable upstream (aka here) or there, so I chose to bother everyone :)

I kind of understand what the autosummary_filename_map configuration option does, but it is not clear to me
if it is supposed to fix my problem and how to properly use it.

Describe the solution you'd like

I'd ask at least for some examples in the documentation (perhaps even inspired by my use case and/or the sciki-learn one.

Start with the documentation for the autosummary_filename_map configuration option and review the linked sphinx-automodapi and scikit-learn examples. Add clear usage examples or a description that explains when and how the option addresses the reported use case, then verify the documentation builds successfully.