docs: Add guide for managing user profile photos (#3280)

[abdulawalarif]

Summary

Addresses serverpod/serverpod#3280.

Adds a procedural guide for uploading, updating, and displaying user profile photos with the v3 auth module. The existing Working with users page covered the edit endpoint API but not the full client flow requested in the issue.

New page: docs/06-concepts/11-authentication/profile-photos.md

• Before you start — auth setup prerequisites and Flutter dependencies
• Expose the profile edit endpoint — UserProfileEndpoint, generated client methods, optional access control
• Upload a profile photo from Flutter — picker helper, setUserImage, error handling
• Display the profile photo — UserProfileModel.imageUrl, CircleAvatar
• Remove or replace a photo — v3 vs legacy behavior
• Configure image size and format — UserProfileConfig and server-side processing
• Configure storage for production — dev URLs, link to file uploads
• Use server-side APIs for custom logic — AuthServices.instance.userProfiles
• Verify and Troubleshooting sections

Updated: docs/06-concepts/11-authentication/03-working-with-users.md

• Adds a link to the new guide under Extending the user profile edit endpoint

Test plan

• Run npm start and open http://localhost:3000/concepts/authentication/profile-photos
• Verify all sections render correctly
• Confirm the link from Working with users to profile-photos resolves
• Confirm links to setup, file uploads, and legacy working-with-users resolve