[www/showcase] Make category filter "accordion" responsive

[fk]

Currently the height of the accordion component containing the "Category" filters/checkboxes is hardcoded to 500px: https://github.com/gatsbyjs/gatsby/blob/master/www/src/views/showcase/collabsible.js#L45-L60

This can result in the accordion not being fully visible. Making one accordion responsive should be easy; however as we might want to add a second accordion (and already do so in the Starter Showcase (#5334), we should investigate how to make a reasonable set (2~5) of accordions responsive. If I'm not totatlly off, flexbox should get us there without additional JS.

Sidenote: If only showing one accordion, do we ever need/should we allow to collapse it?

[swyxio]

this has helped me today: using flexbox with overflow-y: scroll isnt a well documented thing