fixes #16902: remove the .container-fluid from the grid system in modal example since it's doubling up padding

Shouldn't really happen, but China for example has blocked Google so this should help.

Also, include visible <label> as a suggestion in the accessibility
callout.

They're no longer on Folsom St.

[skip sauce]

[skip sauce]

[skip sauce]

X-Ref #13969

Fix #16632

- drop the index pages save for examples
- comment out the extend section for now
- fix up spacing fix on team list

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

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)

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

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

multiple buttons