Overhaul end user documentation

Heavily WIP. This MR contains the first changes to end user documentation. The focus here is on a few main things:

1. Create shorter, more specific content.
2. Make the language easier to read and understand.
3. Maintain a consistent style.
4. Cover more end-user tasks.
5. Use Markdown rather than RST.

There is a lot to do here, so I'm just opening this for now to prompt me to keep plugging away at it.

• Recreate all existing content in new style.
• Add missing content.
• Add mobile UI documentation.
• Remove existing content.
• Create coherent layout for all pages.
• Set up redirects.