Reorganize Diviner articles into `user/` and `tech/`

Summary: Ref T988. I'm splitting the Phabricator documentation into two separate documentation books, one less technical and one more technical. Move all the `.diviner` article files around into `src/docs/user/` or `src/docs/tech/`, accordingly. The only actual changes here are a couple of config changes in the `.book` files.

Test Plan: Regenerated user and technical documentation and saw stuff in the right places.

Reviewers: btrahan

Reviewed By: btrahan

CC: chad, aran

Maniphest Tasks: T988

Differential Revision: https://secure.phabricator.com/D6822