* 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=273306#c34

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

Refs #14839, #19088
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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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If these are supported, why bother mentioning them in a section that's about communicating what's not supported?
These rows made sense when the section also covered IE8 (http://getbootstrap.com/getting-started/#support-ie8-ie9 ),
which doesn't support these features, but we no longer care about IE8 in v4.

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Per http://caniuse.com/#feat=documenthead , document.head is supported in IE Mobile 10+,
so slightly simplify the JS accordingly.

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Fix #18076

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add flexbox to the not supported list 

https://github.com/twbs/bootstrap/issues/17802

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Refs #17237
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Mostly links.
Fixes #17056.

Fix #17055

- Simpler main nav on all pages
- Back to purple masthead on homepage instead of dark graphite
- Active link styles on the main nav
- Cleaned up sidebar nav
- New docs layout name
- Homepage copy edits
- Updated bright purple docs color