doc: update format of pages for ofproto

- add explicit classified layout to ofproto_v1_[23]_ref
- put contents of ofproto_ref back to toctree.

Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>