Re-order popover elements in docs

X-Ref #13969

Include CSS Source Map for bootstrap.min.css

- Update comment as recommended by @cvrebert
- Use `transparent` for `background-color` so we don't need to override that value again in the disabled state.
- Remove disabled changes.

/cc @wonsong

Removes border around <select> caret in IE10+

Clarify that <fieldset> only uses the disabled attribute

Adds @dl-horizontal-breakpoint

Fixed .input-group-addon border-radius

Fix hover border covering active in pagination

Fix #16646

Fix #16632

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.

X-Ref #16583

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
[skip sauce]
[skip validator]

change korean translation site url

[ci skip]

[ci skip]

[ci skip]

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