Review/Improve container thumbnails

[sfc-gh-dmatthews]

A PR (#775) was submitted @abidmix suggesting the API thumbnail for st.sidebar was too abstract and should instead be a screenshot of an example sidebar.

All the container elements have this abstract, schematic look to consistently show the mostly invisible element that is a container, holding other visible elements.

Updating one thumbnail would be a project to update all container thumbnails to maintain consistency. This issue is to track the request and revisit as we routinely update the docs.