docs(router): Document #[end_layout] and layout scoping behavior

[ThomasSteinbach]

Problem

Fixes #627

The router documentation does not explain the stateful scoping behavior of #[layout()], nor does it mention the #[end_layout] attribute. This leads to a common bug where developers accidentally create nested/duplicate layouts.

Root Cause

The #[layout()] attribute creates a persistent, stateful scope that remains active for all subsequent routes until explicitly closed with #[end_layout].

When developers want multiple routes to share a layout, they naturally add #[layout()] to each route, which creates nested layouts instead of a shared layout:

// ❌ WRONG - Creates nested layouts
#[layout(Wrapper)]
#[route("/")]
Home {},

#[layout(Wrapper)]  // This NESTS inside the first layout!
#[route("/about")]
About {},

Result: The /about page renders with two Wrapper components (double header, double footer).

Solution

This PR adds comprehensive documentation explaining:

1. Stateful Layout Scoping

Documents that #[layout()] creates a persistent scope affecting all subsequent routes until closed.

2. The #[end_layout] Attribute

Explicitly documents #[end_layout] (analogous to #[end_nest] for nested routes).

3. Correct Pattern for Multiple Routes

Shows the proper way to apply one layout to multiple routes:

#[layout(Wrapper)]
    #[route("/")]
    Home {},
    
    #[route("/about")]
    About {},
#[end_layout]

4. Warning About Common Mistake

Includes a "Common Mistake" section with a working example showing the wrong pattern and explaining why it produces duplicate layouts.

5. Layout Scoping Rules

Clear bullet points explaining:

• When layout scopes open
• When they close
• What happens with multiple #[layout()] attributes
• Importance of indentation

6. Debugging Tips

Practical debugging guidance:

• Checking HTML output with curl/DevTools
• Using cargo expand to verify macro-generated nesting levels
• Adding #[end_layout] to fix issues

Changes

• docs-src/0.7/src/essentials/router/layouts.md: Added new section "Multiple routes sharing a layout" with subsections
• packages/docs-router/src/doc_examples/outlet.rs: Added two new example modules:
  • multiple_routes: Shows correct pattern with #[end_layout]
  • wrong_pattern: Shows common mistake with nested layouts

Testing

Both new examples include tests that verify the rendered HTML:

• multiple_routes: Renders single layout correctly
• wrong_pattern: Demonstrates the duplicate layout bug (useful for understanding the issue)

Real-World Impact

This bit us in production. Our /tokens page rendered with 2 complete layouts while / rendered correctly. The fix was simple (#[end_layout]), but discovering it required inspecting generated HTML, using cargo expand, and reading Dioxus router source code.

Comparison with Existing Docs

The nested routes documentation already explains this pattern for #[nest()] and #[end_nest]. This PR brings layouts documentation to the same standard.

Alternative Approaches Considered

1. Only document the correct pattern: Rejected - developers need to see the mistake to avoid it
2. Remove #[end_layout] requirement: Not possible - this is core router behavior
3. Add compile-time warning: Would require changes to router macro (out of scope for docs)

The documentation approach is appropriate and follows the pattern used for nested routes.