[DOCS] Users can assign a severity level to a case

[nastasha-solomon]

Description

With elastic/kibana#131694, users can set a case's severity level to any of the following while creating a new case or existing a managing one:

• Low
• Medium
• High
• Critical

From the Cases page, they can filter for a specific severity or choose to view cases of all severities. By default, all severities are shown.

Doc impact

• Need to add this info to procedures for opening a new case.
• Screenshots that need to be updated are mentioned in [DOCS] 8.3 screenshot updates for Cases #1995.