Add release notes scaffold for 9.16. (pgadmin-org#10048)

Create release_notes_9_16.rst with empty sections and add it to the
release notes toctree, so that individual fix/feature PRs can add their
entries without each recreating the file.

Co-authored-by: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: dpage

docs/en_US/release_notes.rst

@@ -12,6 +12,7 @@ notes for it.
    :maxdepth: 1
 
 
+   release_notes_9_16
    release_notes_9_15
    release_notes_9_14
    release_notes_9_13

docs/en_US/release_notes_9_16.rst

@@ -0,0 +1,27 @@
+************
+Version 9.16
+************
+
+Release date: TBD
+
+This release contains a number of bug fixes and new features since the release of pgAdmin 4 v9.15.
+
+Supported Database Servers
+**************************
+**PostgreSQL**: 13, 14, 15, 16, 17 and 18
+
+**EDB Advanced Server**: 13, 14, 15, 16, 17 and 18
+
+Bundled PostgreSQL Utilities
+****************************
+**psql**, **pg_dump**, **pg_dumpall**, **pg_restore**: 18.4
+
+
+New features
+************
+
+Housekeeping
+************
+
+Bug fixes
+*********