[DOCS][8.4] Fallback to @timestamp is configurable when timestamp override is defined

[e40pud]

Issue: elastic/kibana#112315
PR: elastic/kibana#135116

Background

Currently, when a timestamp override field is defined for detection engine rules, @timestamp is always added automatically as a fallback timestamp field. This is useful in cases where multiple data sources are being queried by the same rule, but not all data sources populate the timestamp override field. However, in other cases @timestamp may be completely unreliable and we would like to avoid querying it at all. We should make the @timestamp fallback behavior more configurable, perhaps allowing multiple timestamp overrides to be defined or creating a flag that disables the fallback.

These changes allow user to configure @timestamp fallback when timestamp override is defined. There is a new checkbox which allows to disable fallback to @timestamp when Timestamp Override is defined.

For the 8.4 release, we need to update documentation for the security rule creation. We added a new optional configuration parameter timestamp_override_fallback_disabled in this PR as part of these ticket. The new parameter needs to be added along the timestamp_override which is listed here in the UI docs and here in the create API docs, and also the other APIs (update etc).

We also noticed that timestamp_override is listed in the wrong section there, it should be listed as optional for all rule types, not just EQL, query, and threshold. We should add the timestamp override fallback param in the same places timestamp override appears.

cc @joepeeples

[joepeeples]

Reassigning to @jmikell821 since she's documenting rule creation for 8.4 in #2258.