doc: fix headings in release notes

Fix heading hierarchy in new per-release release notes docs.
Add label to release-notes.rst doc.
Tweak a heading that was picked up as a left-over git marker.

Change-Id: I3e0ec6e1a0c59b05b1a58a32b3484bbeb89cf0f0
Signed-off-by: David B. Kinder <david.b.kinder@intel.com>