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.

Testing done:
Built html docs and checked that links were all correct and that the old
post-review doc was dead.

Reviewed at https://reviews.reviewboard.org/r/4998/