Margin directive

[rowanc1]

JupyterBook allows for margin/sidebar directives (docs). We should have something similar in myst.

Currently we do have a grid system that is defined here:
https://executablebooks.github.io/myst-theme/?path=/docs/components-grid-system--docs

This allows you to add classes, for example to figures to show them in the margins:

:::{figure} images/dc-setup.png
:width: 90%
:class: col-margin-right

Setup of a DC resistivity survey.
:::

To do this we would create a directive:

• There is a start of the directive here. I think it is probably complete actually.
• We need to add an HTML renderer for the directive, similar to the iframe at least as a way to start. We will need to export that and put it into the default renderers so it is picked up.
• The theme should be an <aside> and classes should be col-margin-right probably with zero height h-0 and allow the content to overflow. This should only have zero-height at large enough screen sizes (I think like lg:h-0) as the right margin collapses into the body content on smaller screens.
• Once that is done, I think it will be some more tweaking and start adding out any other options in the directive if there are any!

cc @Jan-David-Black

[rowanc1]

This has been added in #1012.