Document get_user MCP tool

Alex Filatovrequested to merge
596589-implement-get_user-mcp-tool-and-service-docs into master

What does this MR do and why?

This MR documents the new get_user MCP server tool in doc/user/model_context_protocol/mcp_server_tools.md.

The tool itself is added in !253176 (merged). This docs MR follows that MR and should merge after it.

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