docs: add non-Docker Claude Desktop config for MCP

[zaidazmi]

Summary

Adds Claude Desktop configuration examples for running markitdown-mcp without Docker, covering both uvx markitdown-mcp and a pip/virtualenv install using python -m markitdown_mcp.

Fixes #2128.

Validation

• README sanity check confirmed the new uvx, markitdown_mcp, and virtualenv Python path examples are present.
• Docs-only change.

[zaidazmi]

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