Documentation via gitlab pages

This merge request moves the documentation from the readme to a website hosted by gitlab and created from the docs folder in the master branch.

The documentation is hosted at https://training-dataset-dumper.docs.cern.ch. Besides instructions for installation, running and development, it contains an overview of the variables inside the h5 ntuple which is dynamically created from one of the test files as part of the CI. The description of the variables could be improved in subsequent MRs, as well as extended to the other "flavours" of the h5 ntuples. Currently, only PFlow jets are documented.

Further, the MR modifies the .gitlab-ci.yaml file to enable docker image builds for the R22 image again. Unfortunately, some code repetition needed to be introduced for the names of the two base images, as the docker image builder is unable to resolve the DOCKER_CACHE variable.