Added new section for third-party mcp access

[hterminasyan]

What reference architecture does this PR apply to?

outlines the two predominant patterns for third-party MCP access, the governance guardrails each pattern requires, and SAP's recommended approach for production-grade agentic access to SAP solutions.

Who should review your contribution? (Use @mention)

@kay-schmitteckert @anirban-sap @mschfer

Checklist before submitting

• My commits are only for the reference architecture mentioned above.
• I have followed the folder structure in the main README

[cla-assistant]

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0 out of 2 committers have signed the CLA.

❌ Ter-Minasyan
❌ mschfer

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