* Restyle code elements in links

Fixes #21259. Removes the background, color, and padding on code elements within anchors.

* Update _code.scss

* 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 $e...

This pulls in some changes from #18462 to include a set of generic color variables. It doesn't include a Sass map for generating the theme colors yet, since we can't easily do that for all components, but does give a few more color choices to folks and an easier way to customize.

* Overwrite margin-bottom on form-control-static

In the documentation the form-control-static class has been used on a p element.
On the regular vertical form the margin bottom of the p element gets overwritten by a mb-0 class.
In the inline form example this class hasn't been applied, therefore the p element gets a margin-bottom.

To prevent this behavior we can add a margin-bottom of 0.

* Update _forms.scss

* Rename _animation.scss to _transitions.scss for accuracy

* fix docs reference to file name

* cleanup line breaks

Merge branch 'progress-brand' of https://github.com/szastupov/bootstrap into szastupov-progress-brand

Merge branch '14840-consistent-colors' of https://github.com/Kovah/bootstrap into Kovah-14840-consistent-colors

pull->float comment

* Add variables for transitions

* Add variables for transitions

* fix(readme): locked composer to a proper version
Without a version lock, users get current stable

* match docs

Remove mention of popovers auto placement + add note to migration docs

Fixes: #21130