Add screenshots/diagrams to OIDC authentication documentation

Problem

OIDC authentication involves multi-step browser interactions that are not obvious from text alone:

1. Run rucio whoami → get URL
2. Open browser → authenticate at IdP
3. Browser redirects → copy authorization code
4. Paste code → complete authentication

Current documentation: Text-only, no visual guidance
Result: Users get confused, especially at redirect step

Proposed Solution

Add annotated screenshots to docs/user/using_the_client.md:

Visual Workflow Example

Step 1: CLI prompts for authentication

$ rucio whoami
Please authenticate via browser:
https://ri-scale-server.rucioit.cern.ch/auth/oidc_redirect?...

[Screenshot: Terminal with URL]

Step 2: Browser authentication at IdP
[Screenshot: Sample login page with arrows pointing to username/password fields]

Step 3: Authorization code in redirect URL
[Screenshot: Browser URL bar with code parameter highlighted]

https://ri-scale-server.rucioit.cern.ch/auth/oidc_code?
state=...&code=abc123...  ← Copy this code

Step 4: Paste code in CLI

Enter the code: [paste code here]
status     : ACTIVE
account    : root

[Screenshot: Successful authentication output]

Additional Diagrams

Sequence Diagram (for operator docs):

User → CLI: rucio whoami
CLI → Browser: Open auth URL
Browser → IdP: Authenticate
IdP → Browser: Redirect with code
Browser → User: Display code
User → CLI: Paste code
CLI → Rucio: Exchange code (+PKCE)
Rucio → CLI: Return JWT

Advantages

• Reduces confusion: Visual guidance prevents common mistakes
• Faster onboarding: New users succeed on first try
• International: Screenshots transcend language barriers
• Debugging aid: Operators can point users to "this should look like..."

Files to Update

1. docs/user/using_the_client.md - Add workflow screenshots
2. docs/operator/oidc.md - Add sequence diagram
3. Create docs/user/assets/oidc-flow/ folder for images

Start by reading docs/user/using_the_client.md and docs/operator/oidc.md, then review the proposed four-step OIDC workflow. Add annotated screenshots under docs/user/assets/oidc-flow/ and a sequence diagram for the operator documentation; done means both guides visually explain the authentication flow and code handoff.