Add documentation to sso-helpdesk-backend to highlight important code parts

[ay-kay]

• Adjusting the letter template
• Adjusting locations

[rzo1]

Locations

The safest way at this point would be to write in the instructions how to send appropriate SQL statements against the dockerized PostgreSQL database, i.e. delete all locations and insert appropriate ones. This way, schema migrations will go through via Flyway (if we just do it via Flyway, it might break future updates).

As an enhancement, you could later simply create locations via the web interface.

Letter Template

The template can be adjusted in https://github.com/HHN/sso-helpdesk-backend/blob/fba0b964fecaa69201401c58a6684e66a1901fe7/src/main/resources/hhn.xslt and tested with the FopUtil in the test source directory.

As an enhancement, we could make this configurable via the web interface, i.e. a privileged user can just edit the xml inside the web interface. For PDF creation, we fetch the XML from the PostgreSQL database.