Document EESSI environment variables

We set a lot of `EESSI_` environment variables. We should document
- What they are used for
- What valid values are (i.e. which 'format')
- Which ones can be set / overriden by end-users (`_OVERRIDE`) etc.

This will also help ourselves, e.g. to avoid unclarities like we had earlier regarding 'does the `GPU_TARGET_ARCHITECTURE` include `/accelerator` or not?'