[Automated ingestion] Add documentation

[bedroge]

Caspar's review in #232 probably also covers this (partially), but we need some more documentation, both in the code (docstrings, comments) and in a README and/or EESSI docs. Some of #232 (comment) can be reused for this.

[bedroge]

Should also include installation instructions. I think https://github.com/EESSI/filesystem-layer/tree/main/scripts/automated_ingestion may still work for the new code, I'm using the same virtual environment as I used for the old code.