Add MCP Server overview and quickstart

[michaellwest]

Scope

New top-level section mcp-server/README.md with:

• What it is
• Architecture overview (stdio + HTTP transports)
• Installation (NuGet tool package)
• Quickstart (connect to Sitecore, run first script)
• Claude Desktop configuration example
• Link to SitecorePowerShell/mcp-server repo
• Include screenshots for setup steps

References

• SitecorePowerShell/mcp-server (repo)

Files

• New mcp-server/README.md
• SUMMARY.md — Add new top-level section after Remoting