Add role="document" to modal dialog content

to include a note about `role="document"`

Initial fix to generalise use of modals. Generally, true role="dialog"
containers would require only forms/controls as content, with all other
text correctly and explicitly associated via
aria-labelledby/aria-describedby and similar...but as authors use modals
also as generic overlays, this extra role is required to get AT
(particularly NVDA) out of "forms" mode and reenable reading keys

Fix asymmetry between dropup and dropdown carets.

Add Slack link to `CONTRIBUTING.md`

Closes #16599 by merging a tweaked version of it.

Include Slack in CONTRIBUTING.md's list of places to get help

As Slack is a common replacement for IRC for projects (e.g. Wordpress), I propose that we encourage the use of Slack as you have members of the core team using it while IRC often does not have any of the core team available.

generalize dropdowns / drop role="menu"

as role="menu" is a very specific (and strict) ARIA pattern for
desktop-like application menus, and our dropdowns are often used
as pure navigation dropdowns, this change abandons ARIA menus for
a more open-ended and light-weight approach
(see http://heydonworks.com/practical_aria_examples/#submenus and
http://www.w3.org/WAI/tutorials/menus/flyout/#improve-screen-reader-support-using-wai-aria)
note that in dropdown.js, switched to now target ``.dropdown-menu``
instead of ``role["menu"]`` - this also prevents bootstrap scripts
from "bleeding" into non-bootstrap components on the same page.
also removed the ``role=["listbox"]`` part, which appears to be
vestigial/unused (only place in bootstrap that uses that
role are carousels, and their key handling is done separately)

Fixed typo in hide-text.less
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change korean translation site url

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ScrollSpy requires non-body elements to have height and overflow…

[Close #16441] ScrollSpy requires non-body elements to have height and padding

Adjust Carbon Ads margin on -xs

The .container that immediately surrounds .carbonad has a padding of 15px, not 30px.

Fixes #16552.

ensure divider list items have `role="separator"`

http://www.w3.org/TR/wai-aria/roles#separator

Clarify the bs-callout on multiple open modals

Update source maps link

Wall of Browser Bugs: Add link to "jQuery's browser bug workarounds"

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Remove Firefox warning for customizer download.

Closes #15016.

Specifying both the type and URL is deprecated:
* https://docs.npmjs.com/files/package.json#license
* http://npm1k.org/

Closes #16517

[Close #16502] Clarify the bs-callout on multiple open modals

Currently it states overlapping modals, this isn't the only case as this truly applies to any instance when a user would want to open multiple modals at the same time. For this reason, I have modified the title of the callout to make this more clear.

Fixes Bootlint warnings:
	W007 Found one or more `<button>`s missing a `type` attribute.

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