Fixes #10433: Navbar z-index refactor

* Resets the default navbar z-index to auto at a certain breakpoint
* Adds fixed navbar z-index var to fixed bottom navbar (previously only on fixed top navbar)
Fix up padding on default navbar example
Remove default z-index on navbar and only set on static and fixed variants
Merge pull request #11188 from twbs/navbar_zindex_auto

Fixes #10433: Navbar z-index refactor