- 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)

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

Replace the leftover instances with .m-x-auto (and .d-block when necessary)
Closes #19665
Refs #19102
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Strip out .err from our syntax highlighting styles because it fucks up our placeholders in code snippets

flip things around again, no more base class, try out some new stuff to keep responsive flex grid working

- Rename -height to -height-base to match other vars
- Drop use of -height across the board and rely on it to be inherited
- Adjust padding of .dropdown-header to account for different line-height of h6 heading element (this needs refactoring for variables and rems also)

Refs #18782
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So as to decrease the number of future inquiries about said edge case (which is already documented).
Refs #18304
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Things left:
1. clear the selection after copy
2. add messages for success and error

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