Add some documentation about subset collections

[jmcarcell]

BEGINRELEASENOTES

• Add some documentation about subset collections

ENDRELEASENOTES

I didn't see a clear place to put it and it is short enough that it doesn't deserve its own section. I don't add an example in examples.md to avoid duplication. The only documentation that seems to exist now is in https://key4hep.github.io/key4hep-doc/main/how-tos/key4hep-tutorials/edm4hep_analysis/edm4hep_api_intro.html#subset-collections.

Close #858.

[tmadlener]

Thanks a lot. I think it might fit better here: https://github.com/AIDASoft/podio/blob/master/doc/examples.md as advanced topic is quite far down the sidebar on the documentation page.

[jmcarcell]

It's long enough (and even with a subsection about limitations) that I don't think it fits in the examples. The example that is in this PR could be added there maybe, I think that would fit better the existing examples there.

[tmadlener]

Fair point. Can we get a short intro and the example into the examples and a link from there to the advanced topics with the limitations?

[jmcarcell]

Added the small example but left the part in advanced topics untouched, is that what you meant?

[tmadlener]

Yes, exactly. Thank you :)