Document how to update disease (OMIM and HPO) databases

[holtgrewe]

Is your feature request related to a problem? Please describe.
While performing "full" database updates is very important it is not fully documented yet. We are working on this currently for getting also GRCh38 support. However, certain databases such as the HPO and OMIM term databases should be updated more regularly.

Describe the solution you'd like

• Document how to use varfish-db-downloader to download and prepare the file for import into VarFish.
• Document how to import the data into VarFish. Make sure to include information how to verify that the update was correct.

Describe alternatives you've considered
N/A

Additional context
N/A

Discussed in https://github.com/bihealth/varfish-server/discussions/176

^{Originally posted by alexejknaus July 9, 2021}
In order to submit novel variants to ClinVar of novel disorder, it is necessary to provide up-to-date OMIM and HPO IDs.
How can OMIM and HPO databases in VarFish be updated?