[examples]: enhance example with docs, pytest setup, and standalone test server

[gabsuren]

Description

This PR enhances the WebSocket client example documentation and testing infrastructure to improve user experience and development workflow.
The changes add comprehensive documentation, automated testing setup, certificate generation tools, and a standalone WebSocket test server to make the example more accessible and easier to use.

Key Improvements:
Documentation Enhancements:
Added Table of Contents for better navigation
Added Quick Start section with step-by-step instructions
Enhanced certificate generation documentation with multiple options
Improved SDK configuration explanations with pre-configured options

New Features:
Standalone WebSocket server with TLS and mutual authentication support
Automated certificate generation with proper directory structure
Multiple testing approaches (pytest, standalone server, Flask alternative)
Clear usage examples and command-line options

Testing Infrastructure:
Added pytest dependencies and execution instructions
Created standalone WebSocket test server (websocket_server.py) with CLI interface
Separated server code from pytest to eliminate duplication
Added certificate generation script (generate_certs.sh) for automated setup

Checklist
🚨 This PR does not introduce breaking changes.
✅ All CI checks (GH Actions) pass.
✅ Documentation is updated as needed.
✅ Tests are updated or added as necessary.
✅ Code is well-commented, especially in complex areas.
✅ Git history is clean — commits are squashed to the minimum necessary.

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