added more video tutorials

Author: alesanchezr

exercises/04-Class-Selector/README.md

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+tutorial: "https://www.youtube.com/watch?v=RffCSMXgWFc"
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 # `04` Class Selector
 
 There are many ways to select HTML elements to apply styles to them, so far we have only use the `tag` selector, lets talk about the `.class` selector:

exercises/04.1-Combined-Rules/README.md

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+tutorial: "https://youtu.be/watch?v=uQS5QeEE-B4"
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 # `04.1` Combined Rules
 
 CSS files take space on your server and also take time to download (like everything); it is yet another text document that the browser has to download before rendering the page, which is why is important to keep the CSS document as small as possible.

exercises/04.2-Apply-several-classes/README.md

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+tutorial: "https://www.youtube.com/watch?v=da4tJDx48CY"
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 # `04.2` Apply several classes
 
 We have prepared a small CSS Stylesheet that contains the CSS styles to replicate poker cards.

exercises/04.3-id-Selector/README.md

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+tutorial: "https://www.youtube.com/watch?v=9906LCSQJ8"
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+---
+tutorial: "https://www.youtube.com/watch?v=9906LCSQJ8"
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 # `04.3` The ID CSS Selector
 
 You can also select an element by `#id` and apply stiles to it.

exercises/05-Specificity/README.md

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+tutorial: "https://www.youtube.com/watch?v=3JxXkhxyAnI"
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 # `05` Specificity
 
 When creating a CSS document, you need to start from the most generic selector and end with the most specific ones, that is the way CSS works.

exercises/06-Practicing-Rules/README.md

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+tutorial: "https://www.youtube.com/watch?v=4wguurrl-lU"
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 # `06` Practicing Rules
 
 ## 📝 Instructions:

exercises/07-Very-Specific-Rules/README.md

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+tutorial: "https://www.youtube.com/watch?v=2YkLDRZFk40"
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 # `07` Very Specific Rules
 
 # **Important:**