doc: add documentation about the design of media & session management objects

Here is a document that I wrote that documents some concepts about the objects that are involved in the pipewire media and session management graphs. It started off as a wrap-up of what we discussed about the session management objects at the hackfest in November, but to make it more complete I also added some information about other things as well.

When it comes to ALSA UCM, I did change some things. I am not sure if everyone agrees with this design, but it makes more sense to me this way.

Please let me know if you have any objections, comments or even counter-proposals. I can include all ideas in the document and we can try to test different approaches in the implementation.