Use markdown to write more readable documentation

[jlenon7]

This PR changes the documentation file's extension to md and also make use of some markdown features like code blocks to make the documentation more readable.

To see the results, check:

https://github.com/jlenon7/rocket-learn/tree/feat/more-redable-docs https://github.com/jlenon7/rocket-learn/tree/feat/more-redable-docs/docs