Next.js auth tutorial update

[thomas-desmond]

Summary

This PR is an update to the https://developers.cloudflare.com/developer-spotlight/tutorials/fullstack-authentication-with-next-js-and-cloudflare-d1/

closes #21074

Changes include:

• Fix name of file from env.d.ts to new cloudflare-env.d.ts
• Text modifications for clarity
• Code block changes to better support TypeScript
• Make all getCloudflareContext support async

Tested on a Windows 11 machine using Ubuntu WSL.

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.

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PR Change Summary

Updated the Next.js authentication tutorial to enhance clarity and support for TypeScript, including file renaming and async support for context retrieval.

• Renamed file from env.d.ts to cloudflare-env.d.ts
• Improved text clarity throughout the tutorial
• Updated code blocks for better TypeScript compatibility
• Enabled async support for getCloudflareContext function

Modified Files

• src/content/docs/developer-spotlight/tutorials/fullstack-authentication-with-next-js-and-cloudflare-d1.mdx
• src/content/docs/r2/api/s3/presigned-urls.mdx
• src/content/docs/reference-architecture/by-solution.mdx

Added Files

• src/content/docs/reference-architecture/diagrams/storage/storing-user-generated-content.mdx

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