Introduce semantic versioning

[simonmicro]

Currently, we have...

• an old Changelog, which is not really being maintained anymore (also in the wiki)
• a master / next branch (the first should be renamed to main)
• no tags / GH-releases to track changes

So, I propose:

• Add tags using semantic versions (not year.month(.day) pattern like used in the Changelog before)
• Add Github releases for new tags
• Start fresh on version 1.0.0 (I do not want to go back in time and re-version / reevaluate for braking changes everything)
• Update actions to build tagged versions
• Add contributing-guide to the wiki, specifying to work against the next-branch
• Drop the changelog, as it is replaced with tags & releases
• The current docker-tags are just historic reasons at this point (we have no python2 anymore anyways)
  • add -minimal, -full suffix to main, next, [verion-tag] images
  • keep the classic tags as non-documented backward compatibility