Doucmentation improvement: add table for supported plugins for metrics, ranges and reports

[SimeonEhrig]

After the building the project, I was a little bit surprised, that no plugin covers all three functions. If I look in project structure documentation, it makes sense, but a table would be better.

I think the table can be also a good place to note, which supported combinations are WIP/possible and what is not possible.

[j-stephan]

Good idea! Thanks for noting this.