Add a Read the Docs configuration file (fix RTD build job)

[musicinmybrain]

As requested in https://github.com/bids-standard/pybv/pull/105#issuecomment-1736995017, this attempts to fix the Read the Docs job by adding a configuration file (https://docs.readthedocs.io/en/stable/config-file/v2.html).

[musicinmybrain]

I haven’t set up Read the Docs before, but I followed the documentation at https://docs.readthedocs.io/en/stable/config-file/v2.html, and the result looks correct.