docs: Add automatic version numbering for documentation

Because I'm bound to forget to update it at some point otherwise
build: Bump required gtk-doc version to 1.20

We're using Markdown doc comments which are only supported
since gtk-doc 1.20, so bump the required version.
docs: Various documentation fixes and improvements

* Fix cross references
* Fix error in Python example (args the wrong way round)
* Add section explaining caching
* Add a note clarifying the difference between play_simple() and
  play_full() with a NULL callback
* Add a note that the gsound_context_*v() overrides are intended
  to be used by language bindings

For some reason gtk-doc still doesn't mark the GSoundContext class
as implementing the GInitable interface. I have no idea why.