Previously, when running the docs locally, the site, rooted at:
    http://localhost:9001/
would reference docs assets using relative URLs such as:
    /../assets/js/vendor/anchor.min.js
which is equivalent to:
    http://localhost:9001/../assets/js/vendor/anchor.min.js
which is nonsense, since the root directory has no parent directory.
Apparently browsers silently ignore this extra '..', hence why this wasn't noticed until now.
But if you adjust Jekyll's `baseurl` setting, this mistake causes incorrect URLs to get generated.

This commit corrects the problem by removing the extra '../' from the paths.

These paths are also referenced in the Gruntfile, where the fix actually allows us to simplify the code.
Previously, in the Gruntfile, we were doing, e.g.:
    path.join('./docs/assets', '../assets/js/vendor/anchor.min.js')
which calculates to:
    ./docs/assets/../assets/js/vendor/anchor.min.js
which can be simplified to:
    ./docs/assets/js/vendor/anchor.min.js
So we can remove the '/assets' suffix from the left argument
and the '../' prefix from the right argument
and still obtain the same result.

Refs #19990
Continues the degruntification process.

Also removes mq4-hover-shim for now,
since it doesn't yet implement the standard PostCSS plugin interface.

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Fixes #20002

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Refs #19990

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Fixes #19908

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Refs #19908

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Refs #19908

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Refs #19908

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Refs #19908

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Refs #19908.

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Refs #19908.

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Refs #19908

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

Use concat's process function instead.

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Refs http://sauceio.com/index.php/2016/03/new-browser-version-shortcuts/
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Also switch to `^`.

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