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/meson.build

@@ -74,6 +74,7 @@ expand_content_files = [
   'tutorial.md',
   'types.md',
   'signals.md',
+  'value-collection.md',
 ]
 
 gobject_toml = configure_file(input: 'gobject.toml.in', output: 'gobject.toml', configuration: toml_conf)