docs: add Baizhi Agent Toolkit MCP example

[ct-jaryn]

Issue for this PR

Closes #49524

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Adds a Baizhi Agent Toolkit example to the MCP servers documentation using the existing remote transport and an environment-variable Bearer key. It explains account and credit requirements, setting the key before starting OpenCode, and limiting enabled tools.

Disclosure: I am a Baizhi Cloud maintainer.

How did you verify your code works?

• Parsed the new JSON example and checked its fields against the current MCP schema and environment-variable loader.
• Compiled the full page with MDX 3.1.1 and checked formatting with Prettier 3.6.2 using the repository's settings.
• Verified that the original page content is unchanged.

I have not run an authenticated production end-to-end test or a full documentation-site build.

Screenshots / recordings

Not applicable; documentation only.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR