Update example for alertmanager config with templates

[alessandro-verzicco]

What this PR does

Update documentation to make the alertmanager.yml config example coherent. In particular the example shows how to configure alertmanager with a custom alert template using mimirtool, but omits an important part: specifying the template file name in the alertmanager config. This will lead to a mis-configured alertmanager which will not send any notification and logs similar to:

caller=dispatch.go:353 level=error component=MultiTenantAlertmanager user=infra component=dispatcher insight=true component=dispatcher msg="Notify for alerts failed" num_alerts=1 err="slack-infra/slack[0]: notify retry canceled due to unrecoverable error after 1 attempts: template: :1:12: executing \"\" at <{{template \"standard_alert\" .}}>: template \"standard_alert\" not defined"

Which issue(s) this PR fixes or relates to

Fixes #7075

Checklist

• Tests updated.
• Documentation added.
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX].
• about-versioning.md updated with experimental features.

[CLAassistant]

All committers have signed the CLA.

[dimitarvdimitrov]

Thanks for the fix!

[dimitarvdimitrov]

sorry, can you rebase the branch onot grafana:main and push again? I don't know how else to try to get the CI to run. I can't merge the PR unless CI passes

[alessandro-verzicco]

done

[dimitarvdimitrov]

now i know what it was. The new merge "experience ✨" in GitHub doesn't have the "Approve and run" button. I had to switch to the old one to approve the CI. Now CI is running. I'll merge on green. Thanks again!