Add database_documentation and improve schema patch doc.

[horenso]

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[chr-schr]

@markus2330 I separated the improved schema.rs patch documentation into https://github.com/ElektraInitiative/PermaplanT/pull/1232 In the meeting we decided that we'll stick with the current approach (manually updating the database documentation in markdown files), right? Should I write up a decision to stay with the current approach?