[DOCS] Update manual downsampling documentation to use TSDS (#97976) (#98192)

* [DOCS] Update manual downsampling documentation to use TSDS

* Swap manual and ILM downsampling examples in nav

* Typo

* Update prerequisites based on review feedback

* Warn against deleting the old backing index.

* Clarify counter/gauge results

* Mention that the downsampled type is 'aggregate_metric_double'

Author: abdonpijpelink