build website in release pipeline

[markus2330]

As discussed in #3824 the website could be built in the release pipeline, then we do not need to wait for one more master build after the release (if it is possible and makes sense). The process ideally would be:

• wait for a "green" master
• press the release button
• confirm with "yes"
• finished 🚀

If we do it: Please also update the docu for how to do a release accordingly.