docs: Document scope design and AND semantics (#3267)

[abdulawalarif]

Summary

Addresses serverpod/serverpod#3267.

Expands the Authorization on endpoints section in The basics to explain how Serverpod scopes are intended to be used and why multiple scopes are combined with AND logic.

Changes in docs/06-concepts/11-authentication/02-basics.md:

• Clarifies that scopes are independent capabilities with no inheritance (Scope.admin does not imply other scopes).
• Documents the difference between requireLogin and requiredScopes, including that non-empty requiredScopes always requires authentication.
• Adds a How scopes combine subsection with a user-management example (analytics vs edit endpoints) to motivate AND semantics.
• Expands Managing scopes with token snapshot behavior and guidance on revoking tokens for active sessions and streams.
• Updates Custom scopes to tie custom scope constants back to AND evaluation and endpoint inheritance.
• Adds HTTP responses documenting 401 (unauthenticated) vs 403 (insufficient scopes).

Also fixes a missing newline before the User authentication heading.

Test plan

• Run npm start and open http://localhost:3000/concepts/authentication/basics#authorization-on-endpoints
• Verify How scopes combine, Managing scopes, Custom scopes, and HTTP responses render correctly
• Confirm the link to Endpoint inheritance resolves
• Confirm the link to Managing tokens resolves

[Swiftaxe]

Thank you for a good PR on this @abdulawalarif!

I added some comments, mostly nits about improving clarity and simplification.

[Swiftaxe]

1. Custom scopes are introduced in this code example, and below on line 157, before the section "Custom Scopes" which introduces it to the reader.

2. These two examples show creating scopes using a magic string, while the Custom Scopes section teaches the reader to define constants in a class.

Please change the order so Custom Scopes gets introduced first and consider the constant vs magic string examples.

[abdulawalarif]

@Swiftaxe All feedback addressed: em dashes removed, wording simplified, Custom scopes moved before How scopes combine with constant-based examples.

[Swiftaxe]

Thank you, @abdulawalarif! Looks neat!

[abdulawalarif]

Hey, Alexander!
Just checking in, do you need anything else from me, or will you merge it soon?
@Swiftaxe