[DOCS] Relocate thread pools content (#55814)
Moves [thread pools content][0] from [Modules][1] to
[Configuring Elasticsearch][2].
Supporting changes:
* Changes page title to "Thread pools"
* Increments several headings
* Removes several unneeded `[float]` attributes
* Updates the anchors of several headings
Relates to #53307
[0]: https://www.elastic.co/guide/en/elasticsearch/reference/master/modules-threadpool.html
[1]: https://www.elastic.co/guide/en/elasticsearch/reference/master/modules.html
[2]: https://www.elastic.co/guide/en/elasticsearch/reference/master/settings.html
Co-authored-by: debadair <debadair@elastic.co>
Co-authored-by: debadair <debadair@elastic.co>
f26f1021e8