docs: Reorganize the docs navigation

The docs have grown organically and it was time for a reorganization, specifically
helping people understand at a single glance the information
that is available in the docs.

The new docs are organized as follows:
- Core docs (policy and data)
- Use-case specific docs
- Operations
- Support

As part of the reorganization, several files were renamed and/or combined.

Also made an editing pass over what was previously the user-attribute docs
and generalized them to focus on data-replication, an issue that is
now promoted to the core docs.  As part of that, updated the images
and removed the old ones; new image sources are in google slides.

Tweaked the navigation bar to give more space on the left-hand column
so that more of the topics are visible without scrolling.

Signed-off-by: Tim Hinrichs <tim@styra.com>