Fix dropdown example in docs (Fix #20410)

https://bugzilla.mozilla.org/show_bug.cgi?id=504622
Refs #12359

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https://wpdev.uservoice.com/forums/257854-microsoft-edge-developer/suggestions/6508895-dialog-element
Refs #20175

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Import the flexbox utilities into the docs flex stylesheet

Remove unused $size parameter

v4: Fix npm JS directory

Recursive glob action

Contextual background-inverse text color fix

Migration docs: Fix typo in 'Reboot' about units

Update variable usage for input group addon

Instead of the regular border-radius var, use the input ones to match sizing with their associated controls.

Progress bar:fix use variable progress-border-radius instead of border-radius

Docs Tooltip Static Demo uses v3 class names

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[skip validator]

v4: Enable flexbox CSS on flexbox grid page

- 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)

add Edge bug 8282613

- More details on columns and gutters
- Break it into clear sections

Remove mention of base class and fix grid examples