docs: document MCP per-user authentication

[psanghvi17]

Summary

• replace the shared server-credential instructions with mandatory per-caller authentication
• document OAuth protected-resource discovery and required environment variables
• add Dex static-client, callback URI, and epinio-api trusted-peer setup
• add Claude Code configuration and authentication verification commands
• clarify compatibility requirements for other MCP clients

Dependency

This documents the authentication prototype on the epinio/mcp feat/auth branch. Keep this PR in draft until the companion implementation is ready to merge.

Validation

• git diff --check
• Docusaurus server bundle compiled successfully
• local Windows client webpack compilation stalled without an error; CI should complete the full production build

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