docs: Move enum type documentation to Markdown

Helps: #3037

docs/reference/gobject/enum-types.md

@@ -0,0 +1,36 @@
+Title: Enumeration types
+
+# Enumeration types
+
+The GLib type system provides fundamental types for enumeration and flags
+types. Enumerations types are collection of identifiers associated with a
+numeric value; flags types are like enumerations, but allow their values to
+be combined by bitwise or.
+
+A registered enumeration or flags type associates a name and a nickname with
+each allowed value, and the methods [`func@GObject.enum_get_value_by_name`],
+[`func@GObject.enum_get_value_by_nick`], [`func@GObject.flags_get_value_by_name`] and
+[`func@GObject.flags_get_value_by_nick`] can look up values by their name or nickname.
+
+When an enumeration or flags type is registered with the GLib type system,
+it can be used as value type for object properties, using
+[`func@GObject.param_spec_enum`] or [`func@GObject.param_spec_flags`].
+
+GObject ships with a utility called `glib-mkenums`, that can construct
+suitable type registration functions from C enumeration definitions.
+
+Example of how to get a string representation of an enum value:
+
+```c
+GEnumClass *enum_class;
+GEnumValue *enum_value;
+
+enum_class = g_type_class_ref (EXAMPLE_TYPE_ENUM);
+enum_value = g_enum_get_value (enum_class, EXAMPLE_ENUM_FOO);
+
+g_print ("Name: %s\n", enum_value->value_name);
+
+g_type_class_unref (enum_class);
+```
+
+Alternatively, you can use [`func@GObject.enum_to_string`].

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 = [
   "boxed.md",
+  "enum-types.md",
 ]
 content_images = [
 ]

docs/reference/gobject/meson.build

@@ -72,6 +72,7 @@ endif
 # gi-docgen version
 expand_content_files = [
   'boxed.md',
+  'enum-types.md',
 ]
 
 gobject_gir = meson.current_source_dir() / 'GObject-2.0.gir'