Docs: clarify rationale for button group on links

• clarify that putting links in a button group is, in essence, an alternative to using the navigation component (and cross-reference it)
• remove the non-sequitur about using .active...that's irrelevant here, as it's already mentioned in the docs page for .btn. otherwise, this would then also need the whole extra explanation about conveying meaning with aria-current etc, which is already covered in ample detail in the navs page