Flexbox all the time (Drop IE9 support and remove $enable-flex option) (#21389)

* 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

1 files changed, 0 insertions, 52 deletions

diff --git a/docs/getting-started/flexbox.md b/docs/getting-started/flexbox.md
deleted file mode 100644
index f0acc3dcb..000000000
--- a/docs/getting-started/flexbox.md
+++ /dev/null
@@ -1,52 +0,0 @@
----
-layout: docs
-title: Flexbox
-description: Learn how to enable flexbox support in Bootstrap 4 with the flick of a variable or the swap of a stylesheet.
-group: getting-started
----
-
-Flexbox support has finally come to Bootstrap. Opt-in to the new CSS layout styles with the flick of a variable or the swap of a stylesheet.
-
-## Contents
-
-* Will be replaced with the ToC, excluding the "Contents" header
-{:toc}
-
-## What's included
-
-Flexbox support is available for a number of Bootstrap's components:
-
-- The entire grid system (mixins and predefined classes), which switch from `float`s to `display: flex;`.
-- Input groups, which move from `display: table;` to `display: flex;`.
-- The media component moves from `display: table;` and a number of hacky styles to a simple `display: flex;`.
-
-Vendor prefixes are provided in our compiled CSS with [Autoprefixer](https://github.com/postcss/autoprefixer) via Grunt. Some bugs in IE10-11's Flexbox implementation are worked around via [postcss-flexbugs-fixes](https://github.com/luisrudge/postcss-flexbugs-fixes).
-
-## Why flexbox?
-
-In a nutshell, flexbox provides simpler and more flexible layout options in CSS. More specifically, it provides:
-
-- Easy vertical alignment of content within a parent element.
-- Easy reordering of content across devices and screen resolutions with the help of media queries.
-- Easy CSS-only equal height columns for your grid-based layouts.
-
-All these things are possible outside flexbox, but typically require extra hacks and workarounds to do right.
-
-## How it works
-
-If you're familiar with modifying variables in Sass—or any other CSS preprocessor—you'll be right at home to move into flexbox mode.
-
-1. Open the `_variables.scss` file and find the `$enable-flex` variable.
-2. Change it from `false` to `true`.
-3. Recompile, and done!
-
-Alternatively, if you don't need the source Sass files, you may swap the default Bootstrap compiled CSS with the compiled flexbox variation. [Head to the download page]({{ site.baseurl }}/getting-started/download/) for more information.
-
-## Browser support
-
-Enabling flexbox means **reduced browser and device support:**
-
-- Internet Explorer 9 and below do not support flexbox.
-- Internet Explorer 10 has a few known quirks (see the "Known issues" tab in [Can I use...](http://caniuse.com/#feat=flexbox)), requires using a prefix, and only supports the syntax from the old 2012 version of the spec.
-
-Please be extra conscious of your user base when enabling flexbox in your project. Visit [Can I use...](http://caniuse.com/#feat=flexbox) for details on browser support of flexbox.