Further navbar refinement

Alpha 5 is seeing a major navbar update, but it's still not ready for prime time. I'm struggling to figure out a solid way to build this right now that works for all the components, responsive behaviors, and the default/flex variations.

With some of the other Alpha 6 changes I have planned, I'm hoping to get these remaining navbar issues in our next release:

• #18053 (closed): Container adding extra padding for mobile sizes
• #18115 (closed): Revisit light/dark nomenclature revisited
• #18875 (closed): Flexbox support added
• #20116 (closed): Better .navbar-text support
• Migration docs added