Provide an extension walkthrough for new users

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Walkthroughs could greatly help new users get accustomed with what our extension excepts from them and to also introduce them to the commands available. For example:

• Detect if users have already installed manimgl, if not show them a link to the Manim documentation.
• Add a simple sample file with two Manim Cells to let the explore the syntax ##.
• Show how to preview a cell (also tell them that there are shortcuts ;))
• Show them how to use the most important commands.
• At the end, link to our documentation if they want to go more in-depth.

The walkthrough should be self-contained and only feature the highlights, otherwise we just overwhelm users. I estimate that this is going to be one of a bigger PR. Probably more something for the long-run.