Improve Controllers guide structure and examples

[josbeir]

• Add VitePress callouts for key concepts and summaries
• Introduce code groups for render and redirect variations
• Clarify and tighten wording across controller flow, views, and callbacks
• Replace placeholder redirect snippet with a concrete example

[Copilot]

Pull request overview

Updates the CakePHP Controllers guide to improve readability and scannability by restructuring key sections and adding VitePress-specific presentation components (callouts, details blocks, code groups).

Changes:

• Added VitePress callouts (tip/info) and details blocks to highlight key concepts and summaries.
• Introduced code-group blocks for render() and redirect() variants.
• Replaced placeholder redirect/callback snippets with more concrete examples and tightened wording across sections.

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