Update documentation

Given the various changes and additions to the app's structure and features, we should update the public-facing docs to provide a more thorough introduction.

Review the current public-facing documentation alongside the app's changed structure and features, then identify which parts of the introduction are missing. Done means the public docs provide a more thorough introduction that reflects the current app; the issue does not name specific files or tests to run.

Written by the indexing model from the issue text.