Docs: Add some contact info for matroid developers

[lkastner]

I added some developers contact info for the matroid part of Oscar, as these are not very active on slack, so that questions may reach them directly.

ping: @Sequenzer @danteluber @LukasKuehne @bschroter

Please let me know if you don't want to be on there, but I think at least the original designers of this part should be here. Those that also wrote the book chapter on matroids.

Also ping @ulthiel since this was triggered by your question. ;)

[ulthiel]

What about developers of the graphs part? https://docs.oscar-system.org/stable/Combinatorics/graphs/

[lkastner]

What about developers of the graphs part? docs.oscar-system.org/stable/Combinatorics/graphs

I don't understand this question. Sure, I can add them, but there are many other parts without authors in the docs too. This is definitely a different scope than the original PR. The thing that is special about the matroids is that these developers are not very active on slack, which is different for graphs.

[ulthiel]

Don't know. For example at the bottom of https://docs.oscar-system.org/stable/InvariantTheory/intro/ one can find persons responsible for the part. I thought that's a general thing. Maybe not then.

[joschmitt]

I approve this, so you can merge it at your discretion, @lkastner, once you found out whether these people want to be on that list.

[lkastner]

Ping @dcorey2814