Improve our custom MyST plugins

[jaladh-singhal]

In #298, @bsipocz noted:

• being able to wildcard the list of notebooks for the gallery. I like the current explicit approach, but I think it would make sense to be able to also say tutorials/euclid/*md
• alternatively to the wildcard, and this is very IRSA specific, we could also match on the field 'section'.
• we could raise an error for the unrecognised notebooks when using --strict. I'm not sure how complicated this would be.
• have the test cases moved into the plugin directory/future repository; as you suggested a README would suffice to start with as it could both serve as documentation and test
• probably a separate plugin/widget idea; or maybe an add-on feature here: do add a filtering on top of the gallery view. This would definitely be a feature that would also be super useful for upstream.

This will likely require breaking into sub-issues and then triaging as per priorities. But I agree with most of the ideas.

[bsipocz]

I would also add one more idea here: advertise the plugin upstream!