Clarify autoload plugins/extensions on the jsPsych design page

## Summary

The autoload plugins/extensions are listed in a separate accordion section from the optional plugins/extensions. This might make them easy to miss, e.g. if a researcher wants to do a quick scan of the plugins/extensions listed in the optional section to see if the ones they need are listed.

## Fix

We might be better off moving the autoload plugins/extensions to the same section as the optional ones. The downside is that it might take up a lot of the page space and be hard to see everything (autoload + optional) all at once. But the upside is that researchers can just look at a single section to see if the plugins/extensions they need are listed.

To make it even clearer, the autoload packages could also be shown as checkbox elements, but automatically selected and disabled.

Start at the jsPsych design page and inspect how optional and autoload plugins/extensions are currently grouped and rendered. Decide on a clear combined presentation, then verify that researchers can find all listed plugins/extensions and that autoload items are visibly distinguished from selectable optional items.

Written by the indexing model from the issue text.