docs: Move the GObject tutorial to Markdown

Helps: #3037

docs/reference/gobject/gobject-docs.xml

@@ -43,7 +43,6 @@
     <xi:include href="gobject-query.xml" />
   </reference>
 
-  <xi:include href="tut_howto.xml" />
   <xi:include href="tut_tools.xml" />
 
   <chapter id="api-index-full">

docs/reference/gobject/gobject.toml.in

@@ -43,6 +43,7 @@ urlmap_file = "urlmap.js"
 # The same order will be used when generating the index
 content_files = [
   "concepts.md",
+  "tutorial.md",
   "floating-refs.md",
   "boxed.md",
   "enum-types.md",

docs/reference/gobject/meson.build

@@ -38,7 +38,6 @@ if get_option('gtk_doc')
       'glib-mkenums.xml',
       'glib-genmarshal.xml',
       'gobject-query.xml',
-      'tut_howto.xml',
       'tut_tools.xml'
     ],
     html_assets : [
@@ -72,6 +71,7 @@ expand_content_files = [
   'enum-types.md',
   'floating-refs.md',
   'gvalue.md',
+  'tutorial.md',
 ]
 
 gobject_gir = meson.current_source_dir() / 'GObject-2.0.gir'