v4: Navbars

Meta tracking issue for todos on our navbars across the board. Some of this overlaps with our examples mind you.

• #18049 (closed): Navbar text?
• #18053 (closed): Containers add extra padding?
• #17532 (closed): Dropdowns in navbar navs?
• #17250 (closed): No responsive nav love right now
• #18115 (closed): Light vs dark nomenclature needs ironing out

New from me as of 5/10:

• #19280 (closed): Docs navbar has some funky behavior
• #17990 (closed), #18868 (closed): The blown out responsive navbar transitions on expand, but toggles awkwardly on collapse
• #17539 (closed) , #18863 (closed), #18865: Navbar togglers don't respect color scheme
• #18495 (closed): Navbar and page zoom?
• #18599 (closed): Inconsistent use of .active across navs
• #18751 (closed): Input groups are too wide in navbars
• #18855 (closed): Customizing navbar-brand
• #18875 (closed): Flexbox variation is funky

Pulled from @cvrebert's comment:

• Add migration guidance for navbars and old v3 navbar classes (refs #18767 (closed))
• Fix transitions on collapsible navbars (refs #18868 (closed)) Covered above in the list.
• Should .active go on .nav-item, or .nav-link, or either? In any event, needs clearer docs. (refs #18932)