Add description of checkpoint into sgw/cbl glossary for

Add a description of checkpoint into sgw/cbl glossary and include a link in pre-built database page (at least)

https://docs.couchbase.com/couchbase-lite/current/swift/refer/swift-refer-glossary.html

Checkpoint
A “save state” on a Replicator.  It is a document that describes how far in the replication process a given replicator has progressed.  Note that two checkpoints are saved for every replicator:  local and remote.  If these do not agree at the start of replication then it indicates a severe error during the last run, and replication is forced to restart from the beginning.