Add assets retain documentation

[dario-piotrowicz]

Summary

Screenshots (optional)

Documentation checklist

• Is there a changelog entry (guidelines)? If you don't add one for something awesome and new (however small) — how will our customers find out? Changelogs are automatically posted to RSS feeds, the Discord, and X.
• The change adheres to the documentation style guide.
• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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.

[github-actions]

This pull request requires reviews from CODEOWNERS as it changes files that match the following patterns:

Pattern	Owners
/src/content/docs/workers/	@cloudflare/workers-docs, @GregBrimble, @irvinebroque, @mikenomitch, @korinne, @WalshyDev, @cloudflare/deploy-config, @cloudflare/product-owners, @cloudflare/wrangler, @MattieTK, @cloudflare/dev-plat-leads, @vy-ton, @cloudflare/workers-runtime-1
/src/content/docs/workers/static-assets	@irvinebroque, @GregBrimble, @WalshyDev, @cloudflare/deploy-config, @cloudflare/product-owners, @MattieTK, @vy-ton
/src/content/docs/workers/wrangler/	@cloudflare/wrangler, @irvinebroque, @cloudflare/product-owners, @MattieTK, @vy-ton

[github-actions]

Preview URL: https://622de0f2.preview.developers.cloudflare.com
Preview Branch URL: https://dario-wc-5727-retain-assets.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/workers/static-assets/asset-retention/	https://dario-wc-5727-retain-assets.preview.developers.cloudflare.com/workers/static-assets/asset-retention/
https://developers.cloudflare.com/workers/wrangler/configuration/	https://dario-wc-5727-retain-assets.preview.developers.cloudflare.com/workers/wrangler/configuration/
https://developers.cloudflare.com/workers/configuration/multipart-upload-metadata/	https://dario-wc-5727-retain-assets.preview.developers.cloudflare.com/workers/configuration/multipart-upload-metadata/