modal docs: ensure all <button>s have an explicit `type` attribute

[skip sauce]

Add npm install info.

Update github links in footer.

Fixes #13696.
Fixes #13696.
Fixes #14197.
Closes #13718.

https://www.youtube.com/watch?v=L-4VxEtWyRo

Properly fire hidden event when tooltip is destroyed

Fixes #13031.

add aria-labelledby to accordion example to improve accessibility

Scrollspy: ignore invisible list items

Collapse: count `.collapsing` elements as actives; closes #13447

add role="tab" to .panel-heading-s in accordion example to improve accessibility

Handle lack of .width and .height from getBoundingClientRect() in IE8

Make inDom check of tooltip cross document compatible

Fixes #14483

Fixes #12815.
Closes #14363.

Added handling of aria-expanded=true/false to collapse.js, updated
documentation to include advice on making expand/collapse controls
accessible, updated examples and javascript documentation to use
aria-expanded and aria-controls (when targetting single collapsible
element, using ID rather than class selector)

Closes #14147.
Closes #14153.

Fixes #13071

Remove `aria-describedby` attribute later

Ensure that hidden event is sent when content is really hidden (collapsi...

Thanks to @collinanderson
Closes #14567

Revert #13855 due to #14263

Properly hide checkbox and radio inputs in button groups

Add @hnrch02 to Core Team members list

Cleaned up fix for #14511 (input border-radii variables)