Add OOB alerts and alert rule template as asset type

[karenzone]

Related:
#2760

This PR:

• Adds OOB alert rules for Elastic Agent
• Adds alert templates to list of assets included with integrations
• Restructures content for better flow and to replace several notes and admonitions with heads for better scanability and SEO performance

To Do:

• Confirm license requirements in content and subscription page
• List alerts
• Add example(s) of alerts as integration assets

[github-actions]

🔍 Preview links for changed docs

• reference/fleet/alert-templates.md
• reference/fleet/manage-integrations.md

[karenzone]

cc:/ @nimarezainia @MichelLosier @nchaulet @kpollich Here's a draft. Please let me know what you think, and we can iterate.

[nimarezainia]

Thank you @karenzone
For the Elastic Agent ootb rules we need to provide more detail IMO. Examples of some of these alerts (as documented I the issue) and what they provide the user. @MichelLosier has explained them here for the alerts that will be in the package.

I think we need to document the alert name, condition it is looking for and a blurb description for it (which could just be copy+past from the issue).

Regarding the alerts as integration assets: if we have an example to show that would be great. I know that this content is very much dependent on what the package owner adds to their package.