fixes #22007 and fixes #21976 by changing margin strategy for card deck gutters

* fix image stretching due to flexbox
* fix broke text-muted on dark bg
* no img-fluid needed

- add .col-*-auto to the extend in our grid framework mixins so it gets padding
- this means we can avoid the col-12 classes in our docs for the responsive variants

Change the .navbar-toggler to default contextual .navbar-*-color.

<iframe height='265' scrolling='no' title='Bootstrap - .navbar-toggler color' src='//codepen.io/zalog/embed/MJLZOE/?height=265&theme-id=light&default-tab=css,result&embed-version=2' frameborder='no' allowtransparency='true' allowfullscreen='true' style='width: 100%;'>See the Pen <a href='http://codepen.io/zalog/pen/MJLZOE/'>Bootstrap - .navbar-toggler color</a> by Catalin Zalog (<a href='http://codepen.io/zalog'>@zalog</a>) on <a href='http://codepen.io'>CodePen</a>.
</iframe>

Add missing v4 docs for tooltip/popover methods