Makefile.am: Make HTML doc building depend on stamp file

This patch also adds an explicit list of all files written by
sphinx (“docoutput”).

By using an explicit list the build process is more predictable
and will allow us to include the NEWS file again.

Signed-off-by: Michael Hanselmann <hansmi@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>