Document frontend secrets management

https://redmine.teklia.com/issues/10664

Requires frontend#1566 (closed), frontend#1568 (closed), frontend#1569 (closed)

A new Secret access section must be added to the rights management page to document the new access rights on secrets.

The existing Managing secrets on Arkindex page must be rewritten to use the new frontend secrets management page instead.

The admin interface can still be mentioned, but screenshots or step-by-step instructions are not necessary, a link to the general Django admin info is enough.