Document MCP bridge in Remote control section

Adds a short subsection documenting yade-mcp-bridge under the existing Remote control section, as a third parallel to Python prompt and Info provider. The MCP (Model Context Protocol) bridge lets AI agent clients drive Yade through a WebSocket on port 9002, in the same Python namespace as the interactive console.

The bridge is currently distributed as a separate package (github.com/yusong652/yade-mcp). This MR covers the in-Yade interface only; installation and deployment stay in that project's own docs. Submodule integration trajectory is described in the commit message — this MR doesn't ask for any structural change.