Update Jekyll to v2.4.0 and rouge to v1.7.2.
Add labels explanation to readme
Update grid-framework.less

The `when (@index = 1)` guards are not needed cause the mixins already only match on arity.
Examples: Make use of the `footer` tag instead of a `div`.
Fix #14936: Use correct left/right and margin on tooltip arrows
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]
Merge pull request #14945 from twbs/media-pulls-docs

Replace deprecated uses of .pull-left & .pull-right in .media docs
Revert "Use jsDelivr CDN until BootstrapCDN gets updated"

This reverts commit ea23969150e7329dd4e996b844db0282c4d2319d.

BootstrapCDN has now uploaded Bootstrap v3.3.0
grunt
Customizer: listen to the right event for manual file selection in import
Merge pull request #14954 from twbs/customizer-use-change-event

Customizer: listen to the right event for manual file selection in import
grunt
Merge pull request #14949 from twbs/bots-in-contributing

Explain issue/PR bots in CONTRIBUTING.md
Add Wall of Browser Bugs entry for #14409

Closes #14409.
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Merge pull request #14958 from twbs/chrome-css-animation-mem-leak

Add Wall of Browser Bugs entry for #14409
Cover common contributor mistakes in CONTRIBUTING.md

Based on https://github.com/twbs/rorschach/tree/master/docs
Merge pull request #14959 from twbs/contrib-pitfalls

Cover common contributor mistakes in CONTRIBUTING.md
Merge pull request #14917 from twbs/examples-footer

Examples: Make use of the `footer` tag instead of a class.
Merge pull request #14746 from twbs/gems-bump

Update Jekyll to v2.4.0 and rouge to v1.7.2.