[DOCS] Case metrics provided on Cases page

[nastasha-solomon]

General case metrics are added to the Cases page.

• Dev issue: [Cases] Add MTTR metric to the all cases page kibana#131510
• PR: [Cases] Show the Average time to close stat in the all cases page kibana#131909

Will need to add this to section that describes how to manage existing cases.

[nastasha-solomon]

Merged #2050