Resolve "Use simple doc server to deploy DORiE docs"

What does this MR do?

Deploy Sphinx and Doxygen documentation to the new doc server running on hermes: https://hermes.iup.uni-heidelberg.de/dorie_doc/.

A dynamic environment is created for every branch that is pushed to this repository. It has the name of the associated branch and is stopped automatically once the branch is deleted via the on_stop condition. The additional job for stopping the environment deletes the respective folder on the doc server.

Building the documentation has been moved into separate build job build:docs. The documentation deploy jobs run out-of-order, and are executed as soon as the respective build job succeeds.

Additional stuff

• Add Environments to Docker image deploy jobs:
  • docker/devel (for deploys from master)
  • docker/$CI_COMMIT_TAG (for deploys from tags)
• Add Graphviz to base docker image for nice class diagrams in Doxygen output.

Is there something that needs to be double checked?

• Environment can be stopped manually, which executes the deploy:stop_docs job successfully.
• After merge: Environment is stopped automatically via deploy:stop_docs.

Can this MR be accepted?

• Implemented new pipeline
• Tested creating and stopping environments
• Verify the current environment is indeed up: https://hermes.iup.uni-heidelberg.de/dorie_doc/158-use-simple-doc-server-to-deploy-dorie-docs/html/
• Pipeline passing
• Re-built base image
• Added entry to CHANGELOG.md
• Add explicit URL to MathJax CDN
• Squash commits option set

After merge

• Make CI variables protected again

Related issues

Closes #158 (closed)