And add explanatory comments to the Carousel & Dashboard examples regarding holder.js

[skip sauce]

The revert was incomplete.

Closes #15754
[skip sauce]

Condense the dropup example

Fixes #15653 — Add comment which explains that how a responsive image should be centered

Redo of #15657 which I couldn't get to rebase properly after all the merges were done last night.

Refs https://github.com/twbs/bootstrap-blog/commit/1c94b7791f8a24a18f08aba40c38d519b873f2d0
CC: @mdo

[Fix Issue #15701] Tooltip/popover destroy docs don't explain the delegation case.

Add to popovers

[skip sauce]

[Fix Issue #15683] `readonly` input should not have a `not-allowed` cursor. Instead, this pull request displays a standard cursor and only displays the not-allowed cursor on disabled items.

Revise docs wording to clarify cursor display

Fix issue #15601 — Position of X-UA-Compatible, charset and viewport

Add comment to all examples

The non-responsive is correctly saying 2 instead of 3

Add tabindex="-1" to main #content to work around bug in Chrome (and old
WebKits) and IE
Update advice in accessibility section
Include patch in docs CSS to suppress outline when #content is focused
(particularly as Chrome sets focus as result of mouse click for
tabindex="-1" elements)

Stability > Precision

data-example-id not need be moved. Only change, because data-example-id="static-dropdown" already exists.

Remove whitespace after h3.
Variation not need capitalize.

* Remove `<h3>Example</h3>;`
* Move `data-example-id=""` to `<h3>Dropup Variation</h3>` and change value;

Fix typo on Utilize

Surprisingly those weren't reported by the validator; only in the grunt-contrib-htmlmin branch they were reported, so after digging into it, I saw they were indeed errors...

Fix issue #15597 — Mention dropup variation in dropdown docs

Other pull request was so messed up I sincerely have no clue how to save it. The recommended command doesn't work, can't believe rebasing is so problematic...

People normally center images using text-align. However, the `.img-responsive`
class changes the display type to block. As a result you can no longer align the 
image with text-align. You must instead do it with margin: auto. There was no 
note about this on the web page, and no mention about setting the display to
block. Users were left on a frustrating journey to figure out why they can't center
images anymore!

When the screen is shrunk below the md threshold, all the featurette columns stack.
However, since the second featurette is "backwards", when stacked the second featurette
looked awkward as the image came before the heading.

In order to see the changes, you must shrink the browser to below the md threshold.

Use a simple blank.gif and change the background color instead of duplicating stuff.