Write user documentation for control points.

This is to make it as easy as possible to define and use control points.

Without this there is already the design document, examples/ex_control_points.c, and the test developed in WT-13541. That might be enough at first. If so, other MVP tickets, especially WT-13573, should probably be done first.

This jira ticket is the second to the last MVP jira ticket. It collects all the MVP jira tickets except the last one. All MVP tickets except the last one depend upon this jira ticket WT-13575. The last MVP jira ticket is WT-13580.

Note, the control points implementation tickets are in three time periods:

1. POC - Part of the POC.
2. MVP - Part of the minimal viable product. However, even the POC can be used but less conveniently than the MVP.
3. Future - Enhancements after the MVP.

The title of each ticket starts with the time period.