fix: Document FlutterRoute default cache control behavior

[Copilot]

Documentation Update Plan for PR #4454

Based on the changes in serverpod/serverpod#4454, the following updates need to be made to the Flutter web documentation:

• Review current documentation in /docs/06-concepts/18-webserver/09-flutter-web.md
• Add section explaining the default cache control behavior for critical Flutter files
• Document which files get no-cache headers (index.html, flutter_service_worker.js, flutter_bootstrap.js, manifest.json, version.json)
• Document that other files get public, max-age=86400 (1 day) cache headers by default
• Update cache control examples to reflect the new default behavior
• Add explanation that the default behavior is overridable via cacheControlFactory
• Add instructions for invalidating cached assets by bumping Flutter version
• Fix warning to clarify that custom cache control doesn't apply to fallback index.html
• Run markdown linter and fix any issues
• Verify changes are minimal and accurate

Original prompt

Given the changes in this PR: @serverpod/serverpod/pull/4454

Could you update the documentation for the flutter route to cover these changes.

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[SandPod]

@copilot Please add instructions that you should bump the flutter version in order to invalide the cached flutter app.

[Copilot]

@copilot Please add instructions that you should bump the flutter version in order to invalide the cached flutter app.

Added instructions for invalidating cached Flutter assets by bumping the version in pubspec.yaml and rebuilding the app (commit ffbf8e6).