Document incremental backups + PITR that uses them

[deepthi]

We've added an incremental backups feature, and also implemented Point-in-time-recovery that uses them instead of a binlog server. The new PITR has not yet reached parity with the old one, but we still need to document what is available.

Reference PRs: https://github.com/vitessio/vitess/pull/13156 https://github.com/vitessio/vitess/pull/11097