This PR adds 3 documents to the repo:

* `CONTRIBUTING.md` contains the guidelines for contributors how we work.

* `ARCHITECTURE.md` aims to explain the structure of the repository, file naming conventions and external and internal dependencies.

* `STYLE.md` collects naming conventions, code patterns and ux/ui considerations.

Just for the record I would like to clarify: There has been a contributor guide in this repo since 2018. This PR updates the guide.

Question: Is this repo the best place for these documents? Should they apply to plugins as well? Would a central place, like the docs, make more sense?

@afuetterer yes, CONTRIBUTING.md is actually updated, sorry for the confusion.

My idea was that this repo is actually the place where people would look. I expect that ARCHITECTURE.md and STYLE.md evolve with the code and CONTRIBUTING.md remains static. The plan is also that the general assembly of RDMO votes on it, so that is is more official.

We follow KITODO here: https://github.com/UB-Mannheim/kitodo-coding-guidelines.

I implemented all requested changes (I think ...), added a SECURITY.md, and updated the CODE_OF_CONDUCT.md a bit: Mostly about the new Verein, but I also added a paragraph adding a scope:

This Code of Conduct applies within all spaces maintained by the RDMO. Research Data Management Organiser e.V. ("RDMO e.V."), including the GitHub organization at https://github.com/rdmorganiser, the RDMO mailing list, the RDMO Matrix space, and any other communication channels operated on behalf of the community. It also applies when an individual is representing the community in public spaces.

I hope that is ok for everybody.

Still found one typo in STYLE.md line 105:
An exception are fields in models, ...
It should read
"An exception are fields in modals, ..."