Switch library documentation to gi-docgen

Switch from gtk-doc to gi-docgen for the library documentation and
adjust the source files accordingly.

We don't bother about the daemon doc headers as they're not built into
any docs atm.

Closes: #56

doc/meson.build

@@ -1,41 +1,49 @@
 if get_option('gtk_doc')
 
-subdir('xml')
-
-private_headers = [
+expand_content_md_files = [
+  'overview.md',
+  'build-howto.md',
+  'initialization.md',
 ]
 
-content_files = [
-  'overview.xml',
-]
+toml_data = configuration_data()
+toml_data.set('VERSION', meson.project_version())
 
-glib_prefix = dependency('glib-2.0').get_pkgconfig_variable('prefix')
-glib_docpath = join_paths(glib_prefix, 'share', 'gtk-doc', 'html')
-docpath = join_paths(get_option('datadir'), 'gtk-doc', 'html')
+libfeedback_toml = configure_file(
+  input: 'libfeedback.toml.in',
+  output: 'libfeedback.toml',
+  configuration: toml_data
+)
+
+dependency('gi-docgen', version: '>= 2021.1',
+           fallback: ['gi-docgen', 'dummy_dep'],
+           native: true,
+           required: get_option('gtk_doc'))
+
+gidocgen = find_program('gi-docgen')
+
+docs_dir = datadir / 'doc'
+
+custom_target('libfeedback-doc',
+  input: [ libfeedback_toml, libfeedback_gir[0] ],
+  output: 'libfeedback-0',
+  command: [
+    gidocgen,
+    'generate',
+    '--quiet',
+    '--add-include-path=@0@'.format(meson.current_build_dir() / '../src'),
+    '--config=@INPUT0@',
+    '--output-dir=@OUTPUT@',
+    '--no-namespace-dir',
+    '--content-dir=@0@'.format(meson.current_source_dir()),
+    '@INPUT1@',
+  ],
+  depend_files: [ expand_content_md_files ],
+  build_by_default: true,
+  install: true,
+  install_dir: docs_dir,
+)
 
-gnome.gtkdoc('libfeedback',
-           main_xml: 'libfeedback-docs.xml',
-            src_dir: [
-                       join_paths(meson.source_root(), 'libfeedback'),
-                       join_paths(meson.build_root(), 'libfeedback'),
-		       join_paths(meson.build_root(), 'data'),
-                     ],
-       dependencies: libfeedback_dep,
-  gobject_typesfile: 'libfeedback.types',
-          scan_args: [
-                       '--rebuild-types',
-                       '--ignore-headers=' + ' '.join(private_headers),
-                     ],
-       fixxref_args: [
-                       '--html-dir=@0@'.format(docpath),
-                       '--extra-dir=@0@'.format(join_paths(glib_docpath, 'glib')),
-                       '--extra-dir=@0@'.format(join_paths(glib_docpath, 'gobject')),
-                       '--extra-dir=@0@'.format(join_paths(glib_docpath, 'gio')),
-                       '--extra-dir=@0@'.format(join_paths(glib_docpath, 'gi')),
-                     ],
-        install_dir: 'libfeedback',
-      content_files: content_files,
-            install: true)
 endif
 
 if get_option('man')