Update Developer Documentation

[Xaohs]

I have walked through the setup of OAB as a developer from scratch on my (new Arch linux) machine. I have found some possible weaknesses in the developer docs that would be worth adding in (or I might have missed?). Here's my findings:

• Development
  • GDAL is required, and without GDAL you will get an error describing this at the migrate step
  • After installing GDAL, you also need to install/enable postgis as an extension and/or the user that created the database needs to be superuser
  • I don't think anywhere there's specified that you can run cache_zaken to eventually have a "working" environment
  • Translations need to be run initially, this is not documented (src/manage.py compilemessages --locale nl)

@SilviaAmAm Let me know your opinion on which of these should be added to the docs (if any)

[SilviaAmAm]

Yes please document these things! 🙏