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/widget-gallery.md

@@ -0,0 +1,36 @@
+Title: Widget gallery
+
+Widget gallery
+==============
+
+[![Browser](browser.png)](class.Browser.html)
+[![Button](button.png)](class.Button.html)
+[![BusyBox](busy-box.png)](class.BusyBox.html)
+[![ChainButton](chain-button.png)](class.ChainButton.html)
+[![ColorArea](color-area.png)](class.ColorArea.html)
+[![ColorButton](color-button.png)](class.ColorButton.html)
+[![ColorHexEntry](color-hex-entry.png)](class.ColorHexEntry.html)
+[![ColorNotebook](color-notebook.png)](class.ColorNotebook.html)
+[![ColorScale](color-scale.png)](class.ColorScale.html)
+[![ColorScales](color-scales.png)](class.ColorScales.html)
+[![ColorSelect](color-select.png)](class.ColorSelect.html)
+[![ColorSelection](color-selection.png)](class.ColorSelection.html)
+[![ColorProfileComboBox](color-profile-combo-box.png)](class.ColorProfileComboBox.html)
+[![ColorProfileView](color-profile-view.png)](class.ColorProfileView.html)
+[![Dialog](dialog.png)](class.Dialog.html)
+[![EnumComboBox](enum-combo-box.png)](class.EnumComboBox.html)
+[![EnumLabel](enum-label.png)](class.EnumLabel.html)
+[![FileEntry](file-entry.png)](class.FileEntry.html)
+[![Frame](frame.png)](class.Frame.html)
+[![HintBox](hint-box.png)](class.HintBox.html)
+[![IntComboBox](int-combo-box.png)](class.IntComboBox.html)
+[![MemsizeEntry](memsize-entry.png)](class.MemsizeEntry.html)
+[![NumberPairEntry](number-pair-entry.png)](class.NumberPairEntry.html)
+[![OffsetArea](offset-area.png)](class.OffsetArea.html)
+[![PageSelector](page-selector.png)](class.PageSelector.html)
+[![PathEditor](path-editor.png)](class.PathEditor.html)
+[![PickButton](pick-button.png)](class.PickButton.html)
+[![PreviewArea](preview-area.png)](class.PreviewArea.html)
+[![Ruler](ruler.png)](class.Ruler.html)
+[![StringComboBox](string-combo-box.png)](class.StringComboBox.html)
+[![UnitComboBox](unit-combo-box.png)](class.UnitComboBox.html)