Add more notes on custom webhook and content-type for log-based alerting

[aarnq]

Describe the bug

The log-based alerts page only describes a demo with Slack as the notification channel, which is a predefined notifications channel in OpenSearch.

I think we should add more instructions for a the generic custom webhook, as it can be adapted and integrated into applications, and note the particularities that it has.
Such as the bug that the example notification for custom webhooks will be plain text regardless of the set content-type, and that the application developer must provide the a message template that fits the set content-type, as OpenSearch does not provide that.

Additional context
Might be good if we could add information about additional webhooks, even if that means just linking to upstream documentation.