Kill ancient post-review documentation and link people to rbt post.
The Review Board user manual still had a page on post-review, which was mostly

correct but didn't have any information on rbt at all. We also only linked

people to this from the user manual, instead of to our newer rbtools

documentation.
I've removed all content from the old tools/ directory and added links to the

RBTools manual, and cross-linked anywhere that used to link to "post-review" to

"rbt post" in the rbtools documentation.

Built html docs and checked that links were all correct and that the old

post-review doc was dead.