[REQUEST]: Add docs for the new bulk gap filling feature

[denar50]

Description

Summary

The bulk gap filling feature will be available for the user in the bulk action menu as "Fill gaps" for several rules and from the Gaps table as a button that reads "Fill all gaps" for a single rule.

The feature is currently under implementation and the screens provided here might change.

Single rule:

Bulk actions menu

Feature issue: https://github.com/elastic/security-team/issues/10688

Resources

No PR yet.

Which documentation set does this change impact?

Elastic On-Prem and Cloud (all)

Feature differences

The feature should work the same across all deployments.

What release is this request related to?

9.1

Collaboration model

The engineering team

Point of contact.

Main contact: @denar50 (author) / @nkhristinin

Stakeholders: @approksiu / @yctercero