[Workers] Document local observability

[ZinMK]

Summary

Adds a draft Workers local development page for local observability. The page covers local trace and log capture, Local Explorer workflows, Wrangler observability CLI commands, optional hosted MCP access, agent access controls, local schema notes, and troubleshooting.

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.

[cloudflare-docs-bot]

Review

Reviewing new changes (commit fcb6f4d)…