Improve config file documentation

[ptrcnull]

as of now, there's next to no documentation on how the config file structure actually should look

[getzze]

Correct, there is just an example.

It would be important to document because most of the parameters for the providers and refiners are only accessible through the config file...

[ptrcnull]

oh, i couldn't even find the example on https://subliminal.readthedocs.io/en/latest/index.html

[getzze]

ReadTheDocs does not update with new release or commit. I don't know why.
You can build the docs locally and see the example file. Introduced in #1139, it's shown from docs/user/usage.rst