expose ConfluencePublish to BUILD files; remove a lie from page.rst

http://pantsbuild.github.io/page.html#publishing says I can get my org set up with confluence-publishing via a few steps , including adding to a BUILD file...

class OurConfluence(ConfluencePublish):

That was a funny troll, but we should add ConfluencePublish to the register.py so folks really can use it from a BUILD file.

Also, tweak the page.html doc to "include" code that we know compiles OK.

Testing Done:
With this change and with other things hinted at in https://github.com/pantsbuild/pants/issues/385 and https://github.com/pantsbuild/pants/issues/387 , got things set up on my laptop so I could publish to Twitter's wiki. (thus reassuring myself that the change I _wanted_ to make doesn't break confluence publishing, yay)

Furthermore, added a test
https://travis-ci.org/pantsbuild/pants/builds/30708337

Bugs partially addressed: 385

Reviewed at https://rbcommons.com/s/twitter/r/728/