[DOCS] New "Users" landing page

[jmikell821]

Related issue/PR:

• https://github.com/elastic/security-team/issues/2828
• [Security Solution] Changed landing page navigation kibana#129000

Document new "Users" page, where customers can understand authentication and usage information within their environment.

• User host risk score will be behind a feature flag usersenabled, but all customers will be able to see the new Users page.

User Story/ Problem Statement(s)
A security analyst exploring user info expects to see...

• ...total unique users in their organization.
• ...geographic distribution of successful and failed authentications
• ...unique source IP information from successful/failed authentications
• ...host/endpoint information corresponding to user activity
• ...anomalous (ML) unusual user activity
• ...user risk score (https://github.com/elastic/security-team/issues/1637)