docs: Convert the resources page to Markdown

And change mentions of ‘mailing lists’ to ‘support’ while I’m there.

Signed-off-by: Philip Withnall <pwithnall@gnome.org>

Helps: #3037

docs/reference/glib/meson.build

@@ -175,6 +175,7 @@ expand_content_files = [
   'host-utils.md',
   'data-structures.md',
   'programming.md',
+  'resources.md',
   'shell.md',
   'spawn.md',
   'string-utils.md',