docs: add PZERO hosted MCP (Streamable HTTP + Bearer) tutorial

[cursor]

Please add a short tutorial next to the Notion MCP guide (docs/tutorials/integrations/mcp-notion.mdx) for PZERO.

PZERO is a hosted remote MCP for prepaid inference (chat, image, video). Transport is Streamable HTTP. Auth is Bearer pzero_… (not OAuth). Official registry: studio.pzero/mcp. Human docs: https://docs.pzero.studio/agents/mcp

Suggested admin steps (Open WebUI v0.6.31+):

1. Settings > Admin > Integrations > External Tool Servers > Add Connection
2. Type: MCP (Streamable HTTP). Do not pick OpenAPI.
3. URL: https://mcp.pzero.studio/mcp
4. Auth: Bearer. Token: a pzero_… key from https://pzero.studio/agents after top-up.
5. Save. Set Function Calling to Native on the model if tools do not appear.

MCP never moves money itself. Min top-up is 1 USDC on Base.

I cannot open a PR from this account against a public docs repo (no public fork source). Happy to adjust the page if you want a different filename.