[web] Discussion starter about documentation development

With this MR I would like to start a discussion, about making the development of documentation (especially the part of it intended for the people new to the OpenGeoSys ecosystem) more in sync with the development of OGS itself.

My proposition is to consider documentation as a mandatory part of an MR of a new feature before it will be merged.

I have prepared a draft of some guidelines for new contributions to the documentation.