* create responsive containers
provide more flexibility and allow the user to determine when containers switch from fluid to fixed width.
* fix the base container code
this commit fixes the non-media portion of the generated CSS. I learned about the `@extends` directive and was able to put it to good use.
I create a new temporary map that contains all the main `$container-max-widths` and join it to our 2 special cases of 'xs' and 'fluid'. Then we loop through that and, with the appropriate infixes, extend our placeholder
* formatting for style
forgot to run my tests before the last push, i think these are better.
* finish incomplete comment
* fix the responsive containers
using the `@extend` directive I was able to clean up this code
* fix responsive containers in the navbar
mostly we just look through all of our breakpoints so we can include all of the responsive container classes in the tweaks we have to do for the navbar (redeclaring flex properties, don't double up on padding, etc)
* Simplify container extends
* Simplify navbar containers
* Rearrange, add comments, ensure everything is nested in $enable-grid-classes
* Reduce new CSS by using attribute selector
We avoid using `@extend` whenever possible, and this is more readable
* Update _grid.scss
* Update _navbar.scss
* Add docs for responsive containers, redesign the container layout page
* Add to the Grid example
* Remove default kbd element shadow, simplify selector while here
* Add new -shadow-inset variable so our shadows match on .form-control's, .form-select's, and .progress's
* Reuse box-shadow variables for .dropdown's, .popover's, .toast's, .modal's, and .img-thumbnail's
