Replace nonfunctional iframe with thumbnail link

GitHub flavored Markdown disallows `iframes` (see: https://github.github.com/gfm/#disallowed-raw-html-extension-). We therefore need to supply our own thumbnail and link to the video.

Author: AndrzejProchyra

README.md

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 This book is a companion booklet of the Advanced Design Mooc. 
 [https://advanced-design-mooc.pharo.org](https://advanced-design-mooc.pharo.org)
 
-<iframe width="560" height="315" src="https://www.youtube.com/watch?v=3A6tLsf4-q0)" frameborder="0" allowfullscreen></iframe>
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+[![Teaser MOOC "Advanced object oriented design and development with Pharo"](https://github.com/user-attachments/assets/90cb48dd-d28e-4086-8fb4-7fe6c9c6f1c3)](https://www.youtube.com/watch?v=3A6tLsf4-q0)
 
 It contains miniprojects that make you practice different design aspect.