Add a simple MCP-Install-Button wizard to /mcp

[digitarald]

I keep getting feedback that a) devs don't know about VS Code's install buttons and b) steps to creating them. This needs to be a guided experience within our website.

Like https://vscodemcp.com/, but a bit simpler to start with.

cc @connor4312 @sandy081

[ntrogh]

@digitarald We now redirect to the GH MCP registry, where should we host this? Or should this actually be in-product as a subcommand for MCP: List Servers, or right-click in Extensions/MCP view?