Fixes #34353

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Co-Authored-By: Slaven Tomac <slaventomac@gmail.com>

This commit includes all the needed workarounds and most changes from the main branch for everything to work, like:

* removing empty lines in raw HTML that break output
* read .browserslistrc, CSS variables from disk instead of duplicating it
* using Hugo mounts
* using Hugo for the docs CSS/JS
* move ToC Sass code to a separate file while adapting it for Hugo

Thus, this patch makes our npm scripts faster since lint runs on one step and there's no separate docs assets processing.

Backport of #31036 to v4

* Follow-up to #29095

This PR fixes the responsive containers that were added in #29095, originally stubbed out in #25631. Apologies to @browner12 for getting that wrong.

Fixes #25631.

* update navbar as well because we cannot reset all containers uniformly

* Update navbars example to include container-xl example to ensure containers match

* rewrite responsive containers docs, add table of max-widths

* Update container docs
- Move table up to the intro
- Remove the container example because it's actually hella confusing
- Update and link to grid example as a demo instead

* 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

* Prep for v4.3.0

* More updates for v4.3.0.

* rerun to build docs source map

Alternate take to #27683 that builds the modifier class based on min-width, such that list groups are stacked until the class's breakpoint is reached. This is less verbose, matches our primary responsive approach, and prevents an extra class. Unfortunately, I believe supporting flush list groups is too much code here, so I've skipped that and made a note in the docs.

Also added examples generate from our breakpoints data file with a protip for how to do equal width items.

- Move 4.1 docs to 4.2
- Update versions everywhere to 4.1.3 with release script
- Manually bump the shorthand version in package.json
- Add 4.2 to the versions docs page
- Update some redirects
- Fix tests asset URLs
- Bump Nuget and more

With the current docs directory setup, I'm making too many mistakes and have to manually address path changes and directory moves on deploy. This makes for a frustrating experience developing locally and shipping releases. With this PR, we're basically back to the same setup from v3—duplicating the dist directory into our docs directory. Not the most ideal, but very straightforward for me as the release manager.

* Restyle code element, clean up variables

- Removes the padding and background-color
- Changes the color to a brand color ()
- Add new variables to kbd element styles to account for removing the code ones
- Remove overrides that were needed previously

* only break between words, and override it in the pre

* make file inputs 100% wide

* scope custom file changes in input group for sizing

* shorter button labels for responsive

* narrower cards

* button was too wide

* downsize

* fewer links in pagination

* redo cell text

* improve overflow docs

* Fix docs horizontal scrollbar in Components > Modal

* Add overflow to docs example class

* Drop the handleUpdate() variant

While I understand the variant covers the "get a particular plugin instance" case from http://getbootstrap.com/docs/4.0/getting-started/javascript/#programmatic-api it seems weird that we do it here but not everywhere else in the docs (randomly checking other pages, we seem to stick to the documented methods as written on those pages). As this isn't properly explained as is, and as it adds nothing (more of a preference for authors), it would be easier to just drop the variant and stick with the documented method as outlined later on in the same page (avoids having to go off on a "why should you use this instead of that" tangent here).

1. move a couple of them before the more specific ones.
2. change nesting to be under the same parent selector
3. use the ampersand more

The option is set to `always-single-line` in stylelint-config-standard which we extend.

Only applies the margins to .highlight instances within .bd-content, thus negating any need for #23279.

the z-index was only corrected for `.modal.show` which made the modal drop behind the header and backdrop when fading out
simply dropping the `.show` from this selector will correct this issue

* Fix modal overflow with our navbar documentation and fix typo

* Fix z-index backdrop and modal with popover/tooltip example

remove - vars, update docs to generate from data yml files, update mixins, and generate card, button, bg, and color classes from the sass map