Update documentation to reflect change from "Raven" to "University accounts (formerly Raven)"

As a documentation maintainer, 
I would like to replace all mentions of "Raven" with "University accounts (formerly Raven)" and update related content 
so that users receive consistent and current authentication guidance.

Background:

The authentication system previously known as "Raven" is now referred to as "University accounts (formerly Raven)". Documentation across the repository should reflect this updated terminology. Additionally, deprecated documentation sections should include warnings, and instructions for when to use OpenID should be clearly provided, with reference to the official University of Cambridge guidance documentation.

Acceptance Criteria:

• All instances of Raven are replaced with University accounts (formerly Raven) throughout the documentation
• Clear instructions are provided in the documentation specifying when to use OpenID authentication. https://help.uis.cam.ac.uk/service/accounts-passwords/it-staff/university-central-directory/authenticate-users-university ?
• Rename domain docs.raven.cam.ac.uk ?