🖌 Add aside,margin and sidebar directives

[agoose77]

This PR replaces #1010 with an aside directive, renaming margin, e.g.

:::{aside}
This content is an aside / margin note
:::

There is also the margin alias, and sidebar which has a different kind.

[agoose77]

@rowanc1 made a good point regarding the interpretation of the AST for a sidebar/margin. Really, these form "asides" which may have subtypes e.g. sidebar.

I've defined a new aside node that has an optional kind. This allows use to implement new kinds of asides, although I don't even know if we want to support sidebars right now. In this PR, margin and aside are semantically identical.

[rowanc1]

This one might be easier to start from the jupyter-books docs on this and demonstrate it in the page rather than in a demo component?

[agoose77]

I think an aside should be a MyST feature, but I would not be surprised if aside isn't very clear in the demo component. We can enable a grid layout on the MyST demo component if needs be.

Welcome your thoughts here if you think it would be better not to use the myst directive and instead include the aside in the main guide body. But, I think we can fix-up the demo component to make it possible to see, probably.

[rowanc1]

Sorry, I was saying to mimic the jupyter-book docs here. The demo component makes it look squished and isn't the best way to demo this I don't think.

Updating the docs a bit:

Going to merge, and we can come back to improving the docs.

[rowanc1]

Thank you for the tests!

[rowanc1]

🚀