[Workers] Updating note on the main key

[Oxyjun]

Summary

Specifying that the main key in the Wrangler file becomes optional if the Worker is using static assets, and therefore specifies the assets directory in the Wrangler file.

Closes #23746.

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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

[github-actions]

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

Pattern	Owners
/src/content/docs/workers/wrangler/	@cloudflare/wrangler, @irvinebroque, @kodster28, @cloudflare/pcx-technical-writing, @cloudflare/workers-runtime-1

[github-actions]

Preview URL: https://b7d2c9ec.preview.developers.cloudflare.com
Preview Branch URL: https://jun-workers-wrangler-main-clarification.preview.developers.cloudflare.com

Files with changes (up to 15)

Original Link	Updated Link
https://developers.cloudflare.com/workers/wrangler/configuration/	https://jun-workers-wrangler-main-clarification.preview.developers.cloudflare.com/workers/wrangler/configuration/