Extensions Folder in Docs

[BalzaniEdoardo]

Create an extension folder in the docs that contains variations of the models implemented in nemos.

This will serve the following purposes:

• Showcase how to build on top of the library with concrete examples
• Have alpha tester for potential new model functionality: people can use and provide feedbacks on extensions
• Understand which tools are the most requested, this can help decide what belongs in the main package

Having the extension in the docs will at least guarantee that the code runs (docs are built regularly).

Ideally, we could have a disclaimer at the beginning of each extention saying: "if you like this and wish it would be included in the main package, vote for it in the nemos discussion page"