Add a license header in theme instead.

Anchor.js 1.0.0 drops the need to use anchor.css, so we remove it
from the codebase and move any leftover styles over to docs.css.

Closes #16488 by merging it.

Fixes #16470
Closes #16483

Closes #15998
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For uniformity, as we don't seem to use this "Enter <X>" phrasing anywhere else.

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Closes #16372

Closes #16362.
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Refs https://github.com/twbs/bootstrap/issues/16362#issuecomment-96268469
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Closes #16122
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Fixes #16361
Refs #16356
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This reverts commit 229e8889.

ZeroClipboard v2.x drops IE < 9 support.

Mostly a duplication of the *Linked items* section, but for buttons.
Includes a note not to use the standard btn classes.

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Taken from https://github.com/ai/autoprefixer-rails/blob/master/vendor/autoprefixer.js

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