feat: how to use user require approval

[IC-1101asterisk]

Adding some text in how-to to explain how the new feature flag works.

Fixes FL-989

[linear]

FL-989 SSO v3: Admin should validate user

See Notion document: https://www.notion.so/owkin-fdn/SSO-v3-99a6bacb57cd4407a5e4537ae018e469

To try and better specify this feature :

• there should be a new type of users "shadow users"
• shadow users can connect but will be met with a page telling them to wait for approval
• they have to contact admin on their own
• admin will need a new page with shadow users to approve them (or on the already existing page user but what if tons of shadow users ?)

- this should be a new model

- new page "unregistered" users

- new "blank" page => go contact your admin user by slack or email

- SSO system handles differently depending on registered or un registered user