docs: Move migrating-gdbus.xml to Markdown

This copies the test code inline into the Markdown, which means it could
diverge from the test code which is compiled and run as part of the
tests. That needs improving, but that can happen in a subsequent commit.

Helps: #3037

docs/reference/gio/meson.build

@@ -162,7 +162,6 @@ if get_option('gtk_doc')
 
   content_files = [
     'migrating-posix.xml',
-    'migrating-gdbus.xml',
     'gio-querymodules.xml',
     'glib-compile-schemas.xml',
     'glib-compile-resources.xml',
@@ -196,7 +195,6 @@ if get_option('gtk_doc')
     content_files : content_files,
     expand_content_files : [
       'migrating-posix.xml',
-      'migrating-gdbus.xml',
       'gdbus-codegen.xml',
     ],
     html_assets : [
@@ -232,6 +230,7 @@ endif
 # gi-docgen version
 expand_content_files = [
   'migrating-gconf.md',
+  'migrating-gdbus.md',
   'migrating-gnome-vfs.md',
   'overview.md',
 ]