docs: Add new document on how to write bindings in pipewire-rs

I added a document about general guidelines for writing bindings in pipewire-rs.

This goes into detail about the owning-type vs ref-type split (like str vs String) that we've started to use as well as listing methods every type should implement (e.g. as_raw (#60) for using with sys functions).

I also moved existing docs into a new docs/ dir to reduce clutter in the repository root.

Comments on details on the guidelines (e.g. naming, other traits to implement, ..) are welcome.