[DOC] Update to email namespaces in Alerting plugin documentation

[AWSHurneyt]

What do you want to do?

• Request a change to existing documentation
• Add new documentation
• Report a technical problem with the documentation
• Other

Tell us about your request. Provide a summary of the request and all versions that are affected.
On this alerting documentation page, we're referencing the following keystore namespaces for emails.
https://opensearch.org/docs/latest/observing-your-data/alerting/monitors/#authenticate-sender-account

./bin/opensearch-keystore add plugins.alerting.destination.email.<sender_name>.username
./bin/opensearch-keystore add plugins.alerting.destination.email.<sender_name>.password

We'd like to update these to the namespaces below as the ones above are legacy namespaces that will be deprecated in version 3.0, so we don't want to encourage using them.

opensearch.notifications.core.email.<sender_name>.username
opensearch.notifications.core.email.<sender_name>.password

What other resources are available? Provide links to related issues, POCs, steps for testing, etc.