doc: update release notes index to use globbing

Use globbing to find release notes and display them in the expected
reverse-sorted order (newest release first).  Some trickery is needed
because were not using two-digit version/subversion numbers so the list
won't sort naturally. This will eliminate the need to edit the index
page on every release, until we get to subversion 10.

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