[Workers] Add full-stack tag to Waku framework guide

[rmarscher]

Summary

Follow up to #25450. Waku is a full stack React server framework and can be listed here - https://developers.cloudflare.com/workers/static-assets/routing/full-stack-application/

Documentation checklist

I'm not sure that a changelog entry is worthwhile for this tiny change. Thank you!

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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