[Fleet] Doc: Support alert rule template as asset type

[karenzone]

Document alerting rules created without users having to go through specifics of our indices and learning how alerting in stack works. This issue covers two 9.2 features: built-in alerts for Elastic Agent, and Alert rule templates as asset type.

Related:

• [Fleet] Support alert rule templates as a kibana asset type in package-spec kibana#229687
• https://github.com/elastic/ingest-dev/issues/4342

[karenzone]

Phase 1 complete: #3537
Phase 2 in progress: List alerts and details (Add list of agent OOB alert rules with descriptions #3608)

[karenzone]

Issue for Phase 2 for Elastic Agent OOB rules: #3815
Work in progress: #3608