Add Tendem integration

[tendemmcp]

Adds an integration page for Tendem (integrations/tendem.md), a hybrid AI + human task service that Haystack agents can call as a remote MCP server.

• Type: Tool Integration
• Repo: https://github.com/Toloka/tendem-mcp
• Endpoint: https://mcp.tendem.ai/mcp (streamable HTTP, OAuth 2.0 or API-key header)

There is no Tendem-specific Haystack package — the page shows how to reach the hosted server with the existing mcp-haystack MCPToolset / StreamableHttpServerInfo, including a runnable Agent example and a note on keeping approval (i.e. spend) tools out of the model's reach.

The repo and report_issue frontmatter fields are set per the contribution README; pypi is omitted since the service ships no Python package.

[vercel]

@tendemmcp is attempting to deploy a commit to the deepset Team on Vercel.

A member of the Team first needs to authorize it.