Fix loading namespaced doc_fragments from collections (#55249)

* Fix loading namespaced doc_fragments

The syntax for specifying a different fragment name was already
using '.' as a separator, so the code needed to be tweaked to
avoid choking on names like `testns.testcoll.fragname` and
`testns.testcoll.fragname.altvar`.

`get_plugin_class()` returns 'docfragment' for the fragment loader;
mangling `subdir` provides consistent alignment with the normal plugin
directory names and avoids needing special handling of plugin types
with 'module' in the name.

* Add changelog entry

changelogs/fragments/55249-doc_fragments_from_collections.yml

@@ -0,0 +1,2 @@
+bugfixes:
+  - Fixed loading namespaced documentation fragments from collections.