Updating the docs

We want to go through the docs on dqlite.io, update or remove any statements that are out of date, and add basic descriptions of interfaces -- like request types -- that have been implemented in the last couple of years. This is separate from the more comprehensive rewrite that's planned for the future once we have a C client library to document.

## Checklist (WIP)

- [x] Remove out-of-date stuff about checkpointing
- [x] Update docs for ASSIGN request (formerly PROMOTE)
- [x] Document new request types:
- [x] TRANSFER
- [x] DESCRIBE
- [x] WEIGHT
- [x] Document new response types
- [x] Document schema version 1 for EXEC, EXEC_SQL, QUERY, QUERY_SQL
- [x] Update documentation about significance of schema version
- [ ] Clarify documentation about the two addresses passed into the dqlite server API
- [ ] Discussion of snapshotting