[ZT] Add managed OAuth step to self-hosted MCP server guide

[GreenStage]

Summary

The self-hosted MCP server guide (Secure MCP servers > Self-hosted application) was missing the step to enable Managed OAuth on the Access application. Without this, non-browser MCP clients (CLIs, AI agents, SDKs) receive a 302 browser redirect instead of a proper OAuth 401 with WWW-Authenticate headers, effectively blocking them from authenticating.

This PR adds:

• A Dashboard step instructing users to turn on Managed OAuth in Additional settings, with a link to the Managed OAuth reference page.
• The oauth_configuration: { enabled: true } field to the API tab's example request body.

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• If a larger change - such as adding a new page- an issue has been opened in relation to any incorrect or out of date information that this PR fixes.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