This PR changes the $link-text-decoration from none to underline to help improve link discovery. It also includes a fix to the docs sidebar to override that for our desired look. I've kind of missed underlines on our links throughout the docs at least, so this feels like a win-win to me.
With the work done in #30262, this also doesn't affect any existing Bootstrap component. All the components you'd expect will nullify the underline thanks to the changes from that PR.
Fixes #15304 (closed).
Let me know your thoughts @patrickhlauke and @twbs/css-review.
Preview: https://deploy-preview-30389--twbs-bootstrap.netlify.com/