Document recovery point API

Product: Tarantool
Since: 3.8

box.backup.recovery_point.create() is introduced to create
recovery point. Recovery point is point in xlog to which one can
truncate it to produce committed state at the time of point creation. The
recovery point has replica_id, lsn pair which unambiguously specify
a statement in xlog. The statement itself should be included on
truncation.

The result is table:

tarantool> box.backup.recovery_point.create()
---
- lsn: 46
  timestamp: 1782208661.7946
  replica_id: 1
...

Result fields are:

• timestamp - Unix-time of recovery point in seconds. It is less than
  or equal to commit time of recovery point entry and larger than or
  equal to time of recovery point creation API call.
• replica_id - replica_id of recovery point statement in xlog.
• lsn - lsn of recovery point statement in xlog.

Create can also take a label as options parameter:

tarantool> box.backup.recovery_point.create({label = 'label-1'})
- lsn: 46
  timestamp: 1782208661.7946
  replica_id: 1
  label: label-1

Recovery points are persistent and replicated.

New box.backup.info().recovery_points field shows all created recovery
points that resides in xlogs of backup in progress.

tarantool> box.backup.recovery_point.create()
tarantool> box.backup.recovery_point.create({label = 'label-1'})
tarantool> box.backup.start()
tarantool> box.backup.info().recovery_points
---
- - lsn: 1
    timestamp: 1782208906.2023
    replica_id: 1
  - lsn: 3
    timestamp: 1782208908.4776
    replica_id: 1
    label: label-1
...

Requested by @nshy in https://github.com/tarantool/tarantool/commit/73b2b4bff1629c21288258536b5607b02a680994.

Start by finding the existing documentation for box.backup and its info() API, then add the box.backup.recovery_point.create() entry point and the recovery_points field. Document the result fields, optional label, persistence and replication behavior, and the backup-in-progress example shown in the issue.