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.
