Allow sections in tutorials and sidebar to collapse

[EssamWisam]

This PR makes use of Franklin's @@ feature and JavaScript to allow markdown files to have collapsable sections. Given any sequence of markdown elements of length two that take the form:

<markdown heading or list item>

<markdown>

Introducing the feature is possible by wrapping the sections with @@ accordingly:

@@dropdown
<markdown heading or list item>
@@
@@dropdown-content
<markdown>
@@

The resulting UI will make it possible to use the heading/list item to show or collapse the markdown in dropdown-content. This is used in the sidebar and all tutorials.

Thanks for this nice package. Absolutely deserves much wider recognition.

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[EssamWisam]

you also need to know the Franklin @@ magic, at least for consistency with existing tutes.

Not really. The script is there for a reason: it loops over all tutorials in the _literate folder and its idempotent. So all that one has to do after running the tutorial is to run the script (which is actually a notebook for better readibility) and the collpase feature will automatically be introduced.

Not only do you need familiarity with markdown and Literate.jl's formatting of markdown

I like that this a concern even if negligble/conditional. This is why I like notebooks more; they are converted to markdown in a single command and are absolutely readable.

[ablaom]

Not really. The script is there for a reason: it loops over all tutorials in the _literate folder and its idempotent. So all that one has to do after running the tutorial is to run the script (which is actually a notebook for better readibility) and the collpase feature will automatically be introduced.

Got it. Can you add instructions in the README.md that explicate this extra step for a maintainer.