Educate about force merges

## About
Elasticsearch, CrateDB, and probably others know the concept of "[force merges](https://github.com/search?q=org%3Acrate%20force%20merge&type=code)" as quickly explained on [CrateDB Cloud » Automation » Actions](https://cratedb.com/docs/cloud/en/latest/cluster/automation.html#actions).

> **Force merge:** Merges segments on eligible partitions to ensure a specified number.

The CrateDB reference documentation also [has information](https://github.com/crate/crate/blob/f004d839719d1e5c6d0045f72cdaa4972d3993b9/docs/admin/monitoring.rst?plain=1#L384-L387) about `ForceMerge` telemetry information.
```
+-----------------------+---------------------------------------------------------+
| ``ForceMerge`` | Thread pool statistics of the ``force_merge`` thread |
| | pool used when running an ``optimize`` statement. |
+-----------------------+---------------------------------------------------------+
```

There was a wish list item:

> Add example for force merge.

I am wondering if that knowledge is only applicable for CrateDB Cloud?

## References

- https://github.com/crate/cloud-docs/issues/85