docs: add GitHub Copilot CLI installation instructions

[GeekTrainer]

Adds GitHub Copilot CLI installation instructions to the README under the Specific MCP Clients section.

Changes

• Added "GitHub Copilot CLI" section after Cline
• Includes interactive setup (/mcp add) and manual configuration reference
• Links to the standard configuration section to avoid duplication
• Links to official Copilot CLI documentation

Testing

• Verified markdown renders correctly
• Internal link to standard configuration section is valid

[cla-assistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}

[GeekTrainer]

Question - would this fall under a small code exception since it's a small docs update, or will you need a CLA to be signed?

[RandomByte]

Thanks for the contribution.

It would be great if you could sign the CLA. It's possible that there is a pre-existing agreement between Microsoft and SAP regarding code contributions, you might need to consult your Open Source Program Office (or similar) for any questions regarding this.