Update alert template documentation

https://docs.percona.com/percona-monitoring-and-management/get-started/alerting.html#alert-rules

This item should be updated to reflect the current state:

2. Alert templates fetched from Percona Platform, according to the entitlements available for your Percona Account. 

Currently, we are not providing additional templates based on the entitlements for customer's Percona Account. 

One option is to just drop the 2nd part of the statement, "according to the entitlements available for your Percona Account". According to Roma, we are pushing updates so there is some value with Platform integration that we shouldn't lose. 

For context:

https://perconacorp.slack.com/archives/C03E1CS41C6/p1678294077178909