v4: Disabled link cleanup (#34924)

manual backport of #34886

per https://www.w3.org/TR/html-aria/#docconformance

> It is NOT RECOMMENDED to use `aria-disabled="true"` on an `a` element with an `href` attribute.
>
>NOTE
>If a link needs to be "disabled", remove the `href` attribute.

This PR removes the unnecessary `href="#"`, `tabindex="-1"`, and `aria-disabled="true"` from disabled links in both docs pages and examples. `aria-disabled="true"` *is* kept for disabled link-based buttons (that have `role="button"`) as there it's appropriate to use (you *want* to convey to assistive technologies that this thing you're claiming is a button is also disabled at the moment)

Further, the PR extends the "Link functionality caveat" to show the "proper" way (removing `href` and adding `.disabled` class only) to disable a link, but then explains what to do if that's not possible (and then keeps an example with all the traditional `href="#" tabindex="-1" aria-disabled="true"`, but explains clearly that it's not ideal). Same sort of explanation is also added to the pointer event utilities page

site/content/docs/4.6/components/buttons.md

@@ -101,19 +101,23 @@ Disabled buttons using the `<a>` element behave a bit different:
 
 - `<a>`s don't support the `disabled` attribute, so you must add the `.disabled` class to make it visually appear disabled.
 - Some future-friendly styles are included to disable all `pointer-events` on anchor buttons. In browsers which support that property, you won't see the disabled cursor at all.
-- Disabled buttons should include the `aria-disabled="true"` attribute to indicate the state of the element to assistive technologies.
+- Disabled buttons using `<a>` should include the `aria-disabled="true"` attribute to indicate the state of the element to assistive technologies.
+- Disabled buttons using `<a>` *should not* include the `href` attribute.
+
+{{< example >}}
+<a class="btn btn-primary btn-lg disabled" role="button" aria-disabled="true">Primary link</a>
+<a class="btn btn-secondary btn-lg disabled" role="button" aria-disabled="true">Link</a>
+{{< /example >}}
+
+### Link functionality caveat
+
+To cover cases where you have to keep the `href` attribute on a disabled link, the `.disabled` class uses `pointer-events: none` to try to disable the link functionality of `<a>`s. Note that this CSS property is not yet standardized for HTML, but all modern browsers support it. In addition, even in browsers that do support `pointer-events: none`, keyboard navigation remains unaffected, meaning that sighted keyboard users and users of assistive technologies will still be able to activate these links. So to be safe, in addition to `aria-disabled="true"`, also include a `tabindex="-1"` attribute on these links to prevent them from receiving keyboard focus, and use custom JavaScript to disable their functionality altogether.
 
 {{< example >}}
 <a href="#" class="btn btn-primary btn-lg disabled" tabindex="-1" role="button" aria-disabled="true">Primary link</a>
 <a href="#" class="btn btn-secondary btn-lg disabled" tabindex="-1" role="button" aria-disabled="true">Link</a>
 {{< /example >}}
 
-{{< callout warning >}}
-##### Link functionality caveat
-
-The `.disabled` class uses `pointer-events: none` to try to disable the link functionality of `<a>`s, but that CSS property is not yet standardized. In addition, even in browsers that do support `pointer-events: none`, keyboard navigation remains unaffected, meaning that sighted keyboard users and users of assistive technologies will still be able to activate these links. So to be safe, add a `tabindex="-1"` attribute on these links (to prevent them from receiving keyboard focus) and use custom JavaScript to disable their functionality.
-{{< /callout >}}
-
 ## Button plugin
 
 Do more with buttons. Control button states or create groups of buttons for more components like toolbars.