docs(examples): expand file upload example

[SonoTommy]

Summary

This expands the existing file upload example into a more practical guide.

Changes

• keeps the basic c.req.parseBody() example
• adds an example with bodyLimit() for size limits
• adds an example for multiple file uploads with parseBody({ all: true })
• adds a short validation note before persisting uploaded files
• adds related links to parseBody, bodyLimit, and validation docs

I kept the page concise and documentation-focused.

[Copilot]

Pull request overview

Expands the file upload example documentation to provide more practical patterns around parsing multipart form uploads, applying request size limits, and handling multiple files.

Changes:

• Adds a basic upload example that validates the presence of a File and returns file metadata.
• Adds an example showing how to enforce a maximum request size using bodyLimit().
• Adds an example for handling multiple uploads via parseBody({ all: true }), plus a short validation note and related doc links.

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[yusukebe]

LGTM!

[yusukebe]

@SonoTommy This is good! Thanks.

[SonoTommy]

Thanks!