Documentation improvements

### User Story

As a user of Sleeper, I want clear documentation that shows me what I need to know, so that I can interact with the system as easily as possible.

As a developer of Sleeper, I want documentation that's manageable, so that I can keep it up to date and correct.

### Description / Background

We'd like the documentation for Sleeper to be concise, correct, and helpful.

We'd like it to be easy to navigate.

We'd like the guides to take the user through a flow that they will find helpful and want to use.

### Technical Notes / Implementation Details

See sub-issues. We may make notes here on potential issues that have not yet been raised.