Extract ComPWA/report repository

[redeboer]

The documentation build takes a long time by now, because of all the technical reports, even if the notebooks under reports/ are not run. This is annoying in PRs like #287 that only update a bit of info on the main website.

One way to make the DX for the main compwa.github.io page a bit better is to extract the reports/ folder to a separate repository. The URL compwa.github.io/reports will still be the same if the extracted repository is called ComPWA/reports.

Related PRs

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1. FEAT: import TRs from compwa.github.io report#1
   ✨ Feature +1
   
2. DX: remove Technical Reports #296
   🖱️ DX +1
   
3. Activate Read the Docs on ComPWA/report and redirect (⏳ see this build)
4. Create ComPWA/reports repository for redirecting GitHub Pages Done with redirects. Only compwa.github.io/reports/ (with slash) does not work.
5. Enforce PR previews on RTD (see DX: freeze slowest TR notebooks report#21)