Update video thumbnails and links in documentation. Use aka.ms and videos in mslearn

[elbruno]

This pull request updates the video links and thumbnails across multiple lesson files to use new URLs and images. The most important changes include updating the video links and thumbnails for lessons on generative AI, development environment setup, core generative AI techniques, and practical samples.

Updates to video links and thumbnails:

• 01-IntroToGenAI/readme.md: Updated the video link and thumbnail for the introduction to generative AI.
• 02-SetupDevEnvironment/readme.md: Updated the video link and thumbnail for setting up the development environment.
• 03-CoreGenerativeAITechniques/01-lm-completions-functions.md: Updated the video links and thumbnails for text completions, chat, and function calling. [1] [2]
• 03-CoreGenerativeAITechniques/02-retrieval-augmented-generation.md: Updated the video link and thumbnail for retrieval-augmented generation (RAG).
• 03-CoreGenerativeAITechniques/03-vision-audio.md: Updated the video links and thumbnails for vision and audio AI. [1] [2]
• 03-CoreGenerativeAITechniques/04-agents.md: Updated the video link and thumbnail for AI agents.
• 04-PracticalSamples/readme.md: Updated the video links and thumbnails for practical samples, including eShopLite with semantic search, real-time audio, and creative writer agent. [1] [2] [3] [4]Use aka and redirect to mslean videos

[Copilot]

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