Build rtfd docs on every change to docs in trunk

[Jonathan Lange]

rtfd only builds docs when requested. It waits for a web hook post to do so.

testtools's rtfd docs should be updated whenever the trunk docs update.

[Jonathan Lange (jml)]

AFAICT, barring Launchpad growing web hook features, this means we need all committers to have a post_push hook installed, and to write that post_push hook.

[Jonathan Lange (jml)]

Moving to Github fixed this.