docs: add code splitting section

[onmax]

Code splitting is a key Nitro feature but wasn't documented. Added section explaining how route handlers are chunked and how to disable it with inlineDynamicImports.

[vercel]

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[coderabbitai]

📝 Walkthrough

Walkthrough

Two documentation pages were updated to add "Code splitting" explanations describing per-route-handler chunking and on-demand loading, and to expand the inlineDynamicImports Rollup option description with default, bundling behavior, and preset implications.

Changes

Cohort / File(s)	Summary
Routing & config docs docs/1.docs/5.routing.md, docs/3.config/0.index.md	Inserted "Code splitting" sections describing that Nitro creates separate chunks per route handler loaded on demand. Expanded inlineDynamicImports docs to state Default: false, explain single-file bundling when true, and note that some presets may enable bundling by default.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

🚥 Pre-merge checks | ✅ 3

✅ Passed checks (3 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title follows conventional commits format with 'docs:' prefix and clearly describes the main change: adding code splitting documentation.
Description check	✅ Passed	The description is directly related to the changeset, explaining that code splitting documentation was added and detailing what was documented about route handler chunking and the inlineDynamicImports option.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.

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✨ Finishing touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment

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[pi0]

We could just link it to configuration section for changing behavior. Some deployment presets are forced to do this (not as benefit really, it increases cold start actually!), not in any normal conditions it is good idea to enable flag.

[onmax]

Addressed in ef3e460

[pi0]

thnx!