Change nav focus to focus-visible

[mdo]

Fixes #37778.

@julien-deramond @patrickhlauke Is this enough to address this, or do we need to do something else for :focus?

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https://deploy-preview-37784--twbs-bootstrap.netlify.app/

[patrickhlauke]

This solves the first part of #37778

First of all, the outline shouldn't be displayed when the focus is given via the mouse; it should remains given by the keyboard. FWIW we removed this kind of behavior from the buttons not a long time ago.

but the contrast is still way too low (particularly in our docs navigation) ... but that part can be sorted separately