Developer documentation about reports

[madelondohmen]

Changes

This PR adds technical documentation about reports. The documentation contains information about:

• How to write a new report type
• How to write unit tests

Issue link

#2648

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Code Checklist

• [x] All the commits in this PR are properly PGP-signed and verified.
• [x] This PR only contains functionality relevant to the issue; tickets have been created for newly discovered issues.
• [ ] I have written unit tests for the changes or fixes I made.
• [ ] For any non-trivial functionality, I have added integration and/or end-to-end tests.
• [x] I have performed a self-review of my code and refactored it to the best of my abilities.

Communication

• [ ] I have informed others of any required .env changes files if required and changed the .env-dist accordingly.
• [x] I have made corresponding changes to the documentation, if necessary.
• [ ] I have included comments in the code to elaborate on what is not self-evident from the code itself, including references to issues and discussions online, or implicit behavior of an interface.

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Checklist for code reviewers:

Copy-paste the checklist from the docs/source/templates folder into your comment.

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Checklist for QA:

Copy-paste the checklist from the docs/source/templates folder into your comment.