* 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
- Make it an explicit class instead of qualifying with elements (.list-group-item-action)
- Rearrange the entire file for more straightforward flow
- Fix text-decoration bug as a result of the reorder
- Switch from h4's to h5's in docs
- Update docs to merge anchors and buttons sections; clearify usage guidelines there, too
- Nullify #17479 in the process
