[Request] Document new rule_source property for rules in the API schema (DRAFT)

[jpdjere]

🚧 Ticket under construction 🚧
[This is a very rough draft - We need the ticket as placeholder, will expand the ticket with much more detail later]

Description

As part of the Prebuilt Rules Customization epic, we are introducing a new rule_source property to our rules API schema. We need to update our API docs to reflect this changes

Background & resources

• PRs: Multiple
• Issues/metas: [Security Solution] Users can Customize Prebuilt Detection Rules: Milestone 3 kibana#174168
• Point of contact: @jpdjere @banderror
• Test environments: TBD

Which documentation set does this change impact?

ESS and serverless

ESS release

TBD

Serverless release

TBD

Feature differences

None

API docs impact

We are introducing a new rule_source property to our rules API schema

Prerequisites, privileges, feature flags

• Feature flag: prebuiltRulesCustomizationEnabled