Update Jekyll to v2.4.0 and rouge to v1.7.2.
Add labels explanation to readme
Add Wall of Browser Bugs entry for #14868

Closes #14868
Merge pull request #14920 from twbs/safari-print-font-size

Add Wall of Browser Bugs entry for #14868
Update grid-framework.less

The `when (@index = 1)` guards are not needed cause the mixins already only match on arity.
Add Wall of Browser Bugs entry for #14708

Closes #14708
Merge pull request #14922 from twbs/ios-cursor-scroll

Add Wall of Browser Bugs entry for #14708
Use RegExp for button plugin's focus shim

Fixes #14923.
Merge pull request #14925 from twbs/button-focus-use-regexp

Use RegExp for button plugin's focus shim
grunt
Examples: Make use of the `footer` tag instead of a `div`.
Use jsDelivr CDN until BootstrapCDN gets updated
Merge pull request #14930 from twbs/use-jsdelivr-cdn-urls

Use jsDelivr CDN until BootstrapCDN gets updated
Fix the iOS 8 unit test failures

Resolves #14919.
Merge pull request #14931 from twbs/fix-ios-unit-test-failures

Fix the iOS 8 unit test failures
fix typo in comment
Fix #14936: Use correct left/right and margin on tooltip arrows
Replace v3.2.1 reference with v3.3.0 since there wasn't a v3.2.1

Closes #14941
Replace deprecated uses of .pull-left & .pull-right in .media docs

Refs https://github.com/twbs/bootlint/pull/153
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Explain issue/PR bots in CONTRIBUTING.md

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[skip validator]