[BSv5] Provide more reliable (and optional) detection of Bootstrap 5

_Originally posted by @jmichelgarcia in https://github.com/google/docsy/discussions/1555#discussioncomment-6188819_

> It was linked to this PR https://github.com/google/docsy/pull/1459/files
> I had my own definition of `Display Styles` in `_variables_project.scss`

