CI improvements

This PR encompasses several CI improvements:

• Introduces new stages for readability:
  • docker - for building docker images
  • tagging - for tagging docker images
  • e2e - for e2e tests
• Tests stage has been moved before build
  This should help improve development speed. Lint and unitTests will fail earlier on.
• Documentation is now build on every PR and docker images are created for main so we have documentation on the dev server.
• yarn caching has been overhauled.
  Build speeds should be faster.
• Fixed an issue when creating documentation.

I assigned @qamodi for review, because these changes will affect other changes to .gitlab-ci.yml for the other pipelines. But everybody else can review as well :)