Improve expand box ("accordion") visibility

[plumbis]

Spinning out of #473-

expand shortcode blocks are easy to miss The expand shortcode is used to hide blocks of text by default. However, in practice these blocks are really easy to miss when reading or scanning a doc, so I think the content hidden by them very rarely gets viewed.

A specific case where this hurts is when a search result is hidden in one of these blocks. e.g. searching the docs for EnvironmentConfig yields one page result that takes you to docs.crossplane.io/latest/software/install. If you then use your browser to "find on page", you get 0 results. EnvironmentConfig looks to not actually be on that page at all, but it's actually hidden underneath the "feature flags" expand block.

Can the expand blocks be made more visually noticeable?