Add database_documentation and improve schema patch doc.

[horenso]

Basics

• The PR is rebased with current master
• I added a line to changelog.md
• Details of what I changed are in the commit messages
• References to issues, e.g. close #X, are in the commit messages and changelog
• The buildserver is happy

Checklist

• I fully described what my PR does in the documentation
• I fixed all affected documentation
• I fixed the introduction tour
• I wrote migrations in a way that they are compatible with already present data
• I fixed all affected decisions
• I added automated tests or a manual test protocol
• I added code comments, logging, and assertions as appropriate
• I translated all strings visible to the user
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• I created left-over issues for things that are still to be done
• Code is conforming to our Architecture
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• Code is consistent to our Design Decisions
• Exceptions to any guidelines are documented

First Time Checklist

• I have installed and I am using pre-commit hooks
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Review

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[chr-schr]

@markus2330
I separated the improved schema.rs patch documentation into #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?