Doxygen cleanup.

Doxygen cleanup.

- Add subpages definitions in appropriate
  section (User or Dev docs) for doc files
  (*.rst, *.md) that being listed at the top
  level of the generated doc page.
- Generate and add API list to RELEASE doc.
- Fix list_api_changes script to use HEAD
  as the endtag so it doesn't need to be
  changed every release.

Change-Id: Iace7b6433359c6b96869cb1db01facbbcb0ac1e6
Signed-off-by: Dave Wallace <dwallacelf@gmail.com>