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Closes #17291 by finishing the last remaining task.
Also reorders the sections in the "Download" docs.

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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

This reuses the existing URLs in the docs Jekyll configuration rather than hard coding.

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See https://github.com/twbs/bootstrap/pull/17793#issuecomment-146960473

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add flexbox to the not supported list 

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

Closes https://github.com/twbs/bootstrap/issues/17493

Closes #17214.

Related to #17314.

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Refs #17237
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Mostly links.
Fixes #17056.

Fixes #17096

Closes #17091

Fix #17055