[DOCS] Update case priv docs

[nastasha-solomon]

Description

Addresses elastic/kibana#112980.

The Case feature is a top-level feature, similar to the Security app, when setting space privs for a role.

Docs

• https://www.elastic.co/guide/en/security/master/case-permissions.html

Notes

• The majority of the Cases prerequisites topic will need to be revised to account for this change.

[nastasha-solomon]

Since users no longer need to set base privs for the Security feature, need to move instructions for accessing Security features (minus Cases) to Kibana space and index privileges.