[Workers] Add Error 501 Not Implemented documentation

[krys-cf]

Summary

[SPM-3086] Add documentation for HTTP Error 501 (Not Implemented) to the Cloudflare 5xx errors section.

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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