Generate doxygen documentation

Modified the .gitlab-ci.yml:

• build_kaniko_command: builds a docker image for doxygen documentation whenever changes are made in doxy_image/Dockerfile
• deploy-job: triggers on successful merge to master if changes are made in data/stacks/**/*. Runs a python script LHCbDoc.py found in doxy_documentation. Takes the merged stack(s) yml files as input and generates the doxygen documentation. Afterwards, we deploy the generated files to eos. See here for details.
• Access to documentation via https://lhcb-doxygen.web.cern.ch/lhcb-doxygen/project/version. An example can be viewed here: https://lhcb-doxygen.web.cern.ch/lhcb-doxygen/davinci/v46r7