Add initial frontend

based on https://gitlab.hzdr.de/model-data-explorer/model-data-explorer-angular/-/commit/826a722cf47a9367dbe429154b1fcbae6f9cb598

@seyed.takyar this now includes the instructions how to develop with the frontend (we did actually set it up for this already). Let me know, if you have problems!

assigned to @philipp.sommer

enabled an automatic merge when the pipeline for 6fcd49d5 succeeds

merged

mentioned in commit dae60b3c