docs: add v9.15 release notes to toctree

Sphinx ran with -W (warnings as errors) and failed both build-docs and
build-python-package CI jobs because release_notes_9_15.rst was not
referenced from any toctree.

Author: asheshv

docs/en_US/release_notes.rst

@@ -12,6 +12,7 @@ notes for it.
    :maxdepth: 1
 
 
+   release_notes_9_15
    release_notes_9_14
    release_notes_9_13
    release_notes_9_12