[Website]: Document what are the features/permissions required to crate alerts

[lucabelluccini]

Type of issue

None

What documentation page is affected

https://www.elastic.co/docs/solutions/observability/incident-management/create-manage-rules

What happened?

Each type of alert requires to grant permissions and enable specific features.
E.g. Custom Threshold requires to have Logs or Infrastructure privileges

Additional info

No response