docs: expand routing chapter with core concepts

[Taure]

Summary

• Add HTTP methods section covering method atoms, '_' wildcard, and multi-method routes
• Add controller return values section with table of the 5 core return types (json, ok, view, status, redirect) and examples
• Add security section introducing the security function, its return values, and auth_data flow
• Add error routes section showing integer status code routes that override Nova defaults
• Add static file serving section for directory and single-file serving
• Reorder sections so foundational concepts (params, methods, return values) come before grouping and auth
• Minor tweak: add plugins key to route group table

Test plan

• mdbook build succeeds with no errors
• All forward references (views-auth-sessions.md, json-api.md, error-handling.md, plugins.md) resolve to existing chapters
• Return values cross-referenced against nova_basic_handler.erl source
• Security returns cross-referenced against nova_security_handler.erl source
• Static file parsing cross-referenced against nova_router.erl source

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