* descriptions for getting started pages

* descriptions for layout

* add content page descriptions

* more descriptions, updates to some existing ones

* correct site url

* add social stuff to config for twitter cards

* add twitter meta tags; use large image for homepage and regular card for all others

* add the assets

* more site config

* more social shiz to partial, remove existing meta for the partial, remove page title from homepage for simpler if statements

Refs https://bugs.chromium.org/p/chromium/issues/detail?id=454172#c22

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Closes #20165

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Refs #19990
Continues the degruntification process.

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

Refs https://bugs.chromium.org/p/chromium/issues/detail?id=273306#c34

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

Refs #14839, #19088
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Closes #19532
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The modal backdrop positioning bug related to iOS' virtual keyboard doesn't seem to reproduce in iOS 8.0+.
(Possibly as a side-effect of https://bugs.webkit.org/show_bug.cgi?id=153224 )
Refs #9023
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* https://bugs.chromium.org/p/chromium/issues/detail?id=273306
* https://bugs.webkit.org/show_bug.cgi?id=138192
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https://bugs.chromium.org/p/chromium/issues/detail?id=175502
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Refs #19077
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Replace inline HTML with markdown, rewording, and cleanup.
Closes #18984

Refs #18984
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[ci skip]

https://crbug.com/262171 was closed as a duplicate of https://crbug.com/454172

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Remove 'including'.

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