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

[jmikell821]

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

Description

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.

[jmikell821]

Closing as this is a duplicate of #2196.