docs: Move the value collection documentation to Markdown

It needs to be in a separate page because it’s all macros and they have
no type/class associated with them.

Signed-off-by: Philip Withnall <philip@tecnocode.co.uk>

Helps: #3037

docs/reference/gobject/gobject.toml.in

@@ -50,6 +50,7 @@ content_files = [
   "boxed.md",
   "enum-types.md",
   "gvalue.md",
+  "value-collection.md",
 ]
 content_images = [
   "images/glue.png",