Replicate SASS variable table

[gshotwell]

This table is great, and (I think) it's generic to bslib and pyshiny. Can we include it in the style shiny apps?

https://rstudio.github.io/bslib/articles/bs5-variables/index.html#nav-link-padding-y

[gadenbuie]

I think it'd be great to have a similar resource for py-shiny. The table in bslib primarily focuses on Sass variables; these aren't accessible in py-shiny because we don't rely on Sass. For py-shiny, we'd instead focus on the CSS variables, which are often related to the Sass variables, but are definitely not a one-to-one match.