[8.13](backport #3690) [Alerting] Add list of valid connectors to alerting rules docs

[mergify]

Updates stateful docs for #3659.

This change needs to be ported to serverless, too.

TODOs:

• Add list of supported connectors.
• Verify that all connectors in the list are supported for each rule.

  ---

  This is an automatic backport of pull request [Alerting] Add list of valid connectors to alerting rules docs #3690 done by Mergify.

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[mergify]

This pull request is now in conflict. Could you fix it @mergify[bot]? 🙏
To fixup this pull request, you can check out it locally. See documentation: https://help.github.com/articles/checking-out-pull-requests-locally/

git fetch upstream
git checkout -b mergify/bp/8.13/pr-3690 upstream/mergify/bp/8.13/pr-3690
git merge upstream/8.13
git push upstream mergify/bp/8.13/pr-3690