docs: add Claude Code MCP guide

[KeuqS]

Summary

Closes #23.

Add docs/claude-code.md with a step-by-step path from creating a gknowledge project to importing and ingesting papers, registering the MCP server with Claude Code, and using the search_wiki, ask, and add_source tools.

Validation

• Cross-checked the project creation and import labels against the current GTK UI.
• Cross-checked make start, the gknowledge-mcp command, the MCP tool names, and the offline-provider behavior against the repository source and README.
• Documentation-only change; no runtime test is required.

[francescobianco]

Hi @KeuqS, thank you so much for your first contribution to gknowledge! 🎉

A step-by-step Claude Code guide is exactly what #23 asked for, and cross-checking the UI labels, make start, the gknowledge-mcp command and the tool names against the source is much appreciated. We'll review and merge it right away.

We'd also love to welcome you as an official Openapi® contributor. Join the registry at openapi/contributors:

1. Fork the repository.
2. Add your row to the table, in alphabetical order:

   | [![KeuqS](https://github.com/KeuqS.png?size=40)](https://github.com/KeuqS) | [Your Name](https://github.com/KeuqS) | [openapi/gknowledge](https://github.com/openapi/gknowledge) |

3. Open a pull request.
4. If you like, add the badge to your profile README:
   

Thanks again, and if you feel like picking up another one, there are more good first issues waiting. 🙌