List OOjs UI documentation

Thanks to I65fe1a78d0b41 and I0b8051074da39edcac we are publishing
the documentation for OOjs UI. Add a link to it.

Re-applies commit 8db0f41e7c023bbb5bca01a9e2072f82e7ef068a.

Change-Id: I57292e07949b43ea58fc96be68b54453adb5317d