Improve expand box ("accordion") visibility

Spinning out of #473-

expand shortcode blocks are easy to miss
The [expand shortcode](https://docs.crossplane.io/contribute/features/#hide-long-outputs) 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](https://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](https://docs.crossplane.io/latest/software/install/#feature-flags)" expand block.

Can the expand blocks be made more visually noticeable?