Add compat flag for assets

[GregBrimble]

Summary

Sorry it's a bit late! This documents the new compat flag for assets. I'm going to follow up with more sweeping changes to how we're documenting routing in the static assets docs. For now, just documenting this flag.

Documentation checklist

• The documentation style guide has been adhered to.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.
• Files which have changed name or location have been allocated redirects.

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PR Change Summary

Documented a new compatibility flag for asset serving in the static assets documentation.

• Added documentation for the 'Navigation requests prefer asset serving' compatibility flag.
• Clarified the behavior of navigation requests with and without the compatibility flag.
• Outlined the implications of the flag for applications in SPA and 404 page modes.

Added Files

• src/content/compatibility-flags/assets-navigation-prefers-asset-serving.md

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[github-actions]

Preview URL: https://9daf8d6a.preview.developers.cloudflare.com
Preview Branch URL: https://add-compat-flag-for-assets-spa.preview.developers.cloudflare.com

[korinne]

Looks good to me!