We currently list everything as a dependencies, regardless of whether
it actually is; this is a source of confusion for users that read the
GTK documentation.

Gi-docgen has a new "related" key in the project configuration which
allows us to list libraries that are merely related to the namespace
we are documenting; the "dependencies" key is used to document the
actual namespace dependencies, now.

The default development branch for GTK is now "main".

And install the gi-docgen project files for GDK.

The Wayland and X11 references need the gtk-logo.svg and the urlmap.js
extra files.

Fixes: #3905

Matthias Clasen authored Feb 21, 2021 and Emmanuele Bassi committed Mar 11, 2021

Reusing the same one duplicates the standalone
docs, which doesn't quite seem right.

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.