DEV-1100 — Add details about webhooks with ML backends (#1951)

* [docs] Update ML backend docs to be more streamlined

* [docs] Add webhook steps and move preannotation steps

* [docs] Improve ML title again

* [docs] Update webhook docs

* [docs] Update where the envvar needs to be set

* [docs] Add content about the deprecation of completions and training format

* [docs] rewrite interactive pre-annotations section

* [docs] fix ml troubleshooting