Build RTD from `docs` extras

[Helveg]

Describe the work done

The docs were being built from docs/requirements.txt which duplicated maintenance work. Now we use the RTD configuration file to specify to install the .[docs] extras.

---

📚 Documentation preview 📚: https://bsb--808.org.readthedocs.build/en/808/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.77%. Comparing base (b623f37) to head (e5f2112).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #808 +/- ## ======================================= Coverage 83.77% 83.77% ======================================= Files 105 105 Lines 13016 13016 ======================================= Hits 10904 10904 Misses 2112 2112 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.