Stop using gtk-doc, and switch to gi-docgen.

The gi-docgen tool generates API references through the introspection
data, which has various benefits:

 - it does not parse C code
 - it does not generate and run C code to introspect types at build time
 - it does not present a different API from the one we're exporting

Additionally, gi-docgen:

 - does not generate DocBook XML in order to generate HTML
 - does not go through xsltproc
 - parses proper Markdown

Which makes it markedly faster than gtk-doc has ever been.