[Integrations] Centralized doc page for the alert rule templates

[alaudazzi]

From a Slack thread with @daniela-elastic (PM on Obs Integrations):

"We will really need some documentation help with a centralized doc page for the alert rule templates that the individual integration pages can point to. A page that explains to users what these templates are and how to use them. Tom has created the automation where the individual integration doc page picks up the alert rules and points to this centralized doc page. What we need is:

• A doc writer to review the new section in the individual integration doc pages, related to alert rules - there's some common text plus some of the content for each alert rule template
• A doc writer to update the centralized doc page with explanation of what the alert rule templates are and how to use them.

[alaudazzi]

Closing in favour of #3678.