docs: Migrate from gtk-doc to gi-docgen

gtk-doc has been slowly dying for the past few years; with gi-docgen we
have a nice successor.

This also makes sure the C documentation also uses the GIR file, which
in turn means faster build times (since all the C code doesn't have to
be parsed and recompiled again), and has a clear dependency graph.

See the [gi-docgen tutorial] for more info on how the system works.

[gi-docgen tutorial]: https://gnome.pages.gitlab.gnome.org/gi-docgen/tutorial.html

devel-docs/reference/gimp-ui/Makefile.am

@@ -0,0 +1,26 @@
+## Process this file with automake to produce Makefile.in
+
+GIMP_DOC_LOGO = $(abs_top_srcdir)/data/images/gimp-logo.ong
+
+gimp-ui-3.0.toml: gimp-ui-3.0.toml.in
+	$(AM_V_GEN) sed \
+		-e 's/@GIMP_VERSION[@]/$(GIMP_VERSION)/' \
+		-e 's/@GIMP_LOGO[@]/$(GIMP_DOC_LOGO)/' \
+		$< $@
+
+# Markdown content files
+content_files = \
+	widget-gallery.md \
+	$(NULL)
+
+GimpUi-reference: $(top_builddir)/libgimp/GimpUi-@GIMP_API_VERSION@.gir $(top_builddir)/libgimp/Gimp-@GIMP_API_VERSION@.gir $(content_files) gimp-ui-3.0.toml
+	$(GI_DOCGEN) \
+	generate \
+	--quiet \
+	--fatal-warnings \
+	--config=gimp-ui-3.0.toml \
+	--output-dir=$(abs_builddir)/Gimp-3.0 \
+	--no-namespace-dir \
+	--content-dir=$(abs_srcdir) \
+    --add-include-path=$(top_abs_builddir)/gimp \
+	$<