Updating the docs

[cole-miller]

We want to go through the docs on dqlite.io, update or remove any statements that are out of date, and add basic descriptions of interfaces -- like request types -- that have been implemented in the last couple of years.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• [x] Remove out-of-date stuff about checkpointing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
• [ ] Document new response types
• [ ] Document schema version 1 for EXEC, EXEC_SQL, QUERY, QUERY_SQL
• [ ] Update documentation about significance of schema version
• [ ] Clarify documentation about the two addresses passed into the dqlite server API
• [ ] Discussion of snapshotting