PMM-13296 - Document additional Percona Alert rule templates

[sleto-it]

• Some initial commits to solve and improve a few things.

and, fix for

• https://perconadev.atlassian.net/browse/PMM-13296

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[rnovikovP]

@catalinaadam can we move this page inside Percona Alerting in structure? it looks logical as this details of the Templates, not a separate section

[catalinaadam]

@catalinaadam can we move this page inside Percona Alerting in structure? it looks logical as this details of the Templates, not a separate section

@rnovikovP - moved it