doc: Install all release notes

This fixes a bug. The 1.4.0-rc1 release installed the 1.3 release notes
but not any 1.4 release notes.

To fix this, let's follow the precedent set by git.  Update CMake to
install *all* release notes.

Signed-off-by: Chad Versace <chad.versace@linux.intel.com>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Emil Velikov <emil.l.velikov@gmail.com>