[Fleet] Doc: Expand Elastic Agent OOB rules content with rules and descriptions

[karenzone]

New issue for Elastic Agent OOB: Phase 2 so that we can close #2760 as complete.

Topic: https://www.elastic.co/docs/reference/fleet/alert-templates

Related: #2760
Follow-up to #3537
Work in progress: #3608

With #3537, we merged docs for core functionality while the dust settled on rules. Merge of PR #3608 will close this issue.

[karenzone]

Closed with merge of #3608.
Published content: https://www.elastic.co/docs/reference/fleet/alert-templates#ea-alert-rules