* remove the $enable-flex variable option

* remove bootstrap-flex.css dist file and it's grunt task

* remove the separate flex css file for docs; it's all the same now

* remove flexbox docs (porting some to the main grid docs in next commit)

* clean up few grid docs bits to simplify copy, start to mention flexbox

* port relevant flexbox-grid.md content to grid.md

- clean up mixins
- update how it works section
- bring over sizing and alignment sections

* remove the $enable-flex from the options.md page

* update lead paragraph to mention flexbox

* update migration to mention loss of ie9 support

* remove mention of flexbox dist file

* clarify IE support

* making a note

* remove flexbox variant mentions from component docs

- updates docs for media object, navs, list group, and cards to consolidate docs
- no more need to callout flexbox variants since it's now the default

* remove $enable-flex if/else from sass files

* remove flex dist files

* update scss lint property order to account for flex properties

* linting

* change to numberless classes for autosizing, wrap in highlighting div

* bump gruntfile and postcss to ie10

* redo intro sections

* rearrange

* phew, redo hella grid docs

- rearrange all the things
- consolidate some bits

* remove reference to flexbox mode

* more border action for demo

* Make some changes to the .card's in .card-deck's to ensure footers align to the bottom

Update gruntfile to compile the Sass extras (flex, grid, and reboot) as part of the dist-css task (#20682)

- Creates new flexbox grid Sass file in our docs assets
- Updates the Gruntfile to compile said new Sass file and minify the output
- Update notice on flexbox docs page for how it works
- Only enable compiled flexbox grid CSS in hosted docs site, not in dev (for easier and specific debugging of all flexbox features)

[skip sauce]

Only grunt-saucelabs is left in order to update grunt to v1.x.

* bower.json, package.json: Extend jQuery version ranges to include v3
* NuGet: Bump jQuery to v3.0.0.1
* Docs+Examples: Update jQuery to v3.0.0
* Use jQuery v3.0.0 for JS unit tests
* Update jqueryVersionCheck to allow jQuery v3.x.x

Previously, when running the docs locally, the site, rooted at:
    http://localhost:9001/
would reference docs assets using relative URLs such as:
    /../assets/js/vendor/anchor.min.js
which is equivalent to:
    http://localhost:9001/../assets/js/vendor/anchor.min.js
which is nonsense, since the root directory has no parent directory.
Apparently browsers silently ignore this extra '..', hence why this wasn't noticed until now.
But if you adjust Jekyll's `baseurl` setting, this mistake causes incorrect URLs to get generated.

This commit corrects the problem by removing the extra '../' from the paths.

These paths are also referenced in the Gruntfile, where the fix actually allows us to simplify the code.
Previously, in the Gruntfile, we were doing, e.g.:
    path.join('./docs/assets', '../assets/js/vendor/anchor.min.js')
which calculates to:
    ./docs/assets/../assets/js/vendor/anchor.min.js
which can be simplified to:
  ...

Refs #19990
Continues the degruntification process.

Also removes mq4-hover-shim for now,
since it doesn't yet implement the standard PostCSS plugin interface.

[skip sauce]
[skip validator]

Refs #19990

[skip sauce]
[skip validator]

Refs #19908

[skip sauce]
[skip validator]

️️️ https://github.com/twbs/bootstrap/graphs/contributors

Use concat's process function instead.

Also remove the workaround for the old validator version.

[skip sauce]

Remove csscomb (soon replace it with something else) because scsslint has most of it covered already

Workaround for https://github.com/jzaefferer/grunt-html/issues/86