Better UX and documentation for SM/SSC conversion

[garcia]

We should document simfile.convert with a little more grace than what the AutoAPI page currently has to offer.

The dedicated page should include examples for practical use cases, like saving to both the SM and SSC file in a simfile directory, validating that the two files on disk have matching base properties, and backfilling missing SM files for the sake of StepMania builds that don't support SSC (like NotITG).