docs: Add user documentation

Existing documentation is rather detailed on the wire protocol, but
lacks a higher level technical overview of the features and
operating principles of the app.

This commit adds two new high-level guides for technical readers
that are not interested in the details of the wire protocol:

- features.md: high-level overview of the app's features
- integration.md: integration concepts and security model

Also, wallet.md was trimmed to avoid repeating what is in BIP-388.