Reorganize the admin section using a Divio-like organizational scheme

[nathandyer]

This is a subset of the larger goal to reorganize and expand our documentation, using an organizational scheme similar to the Divio documentation system.

This issue is meant to serve as a meta-issue to be able to reference other existing issues that are related to this effort, and to help track the progress.

A complete overhaul requires UX consideration, user research and feedback, and a measured approach. It's also important that all existing links remain in place, so that information we've previously provided remains accessible at the original location.

Expanding and reorganizing the Source and Journalist sections will be their own units of the work, along with any new or alternate personas that we determine are worth adding as part of the deeper UX work. It's also worth noting that a strict Divio implementation is likely not ideal for the project, given SecureDrop's strong split between the different user types / personas, and the unique documentation needs for each of them (journalists, admins, sources, etc.).

Related issues: #43