- add more details on custom checkboxes and radios for how we handle the states and how the css works
- create a disabled custom checkboxes and radios example

* https://bugs.webkit.org/show_bug.cgi?id=152637
* http://openradar.appspot.com/24030853

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As per our browser compat policy, we only claim support for latest
versions of Opera on desktop. Current Opera is version 34. From testing, IE11 does support `pointer-events:none` just fine now.

content had already been wrapped in the `<div class="col-md-3 col-md-push-9 bd-sidebar">`

to harmonise it with the callout given in pagination

Remove 'including'.

"Go somewhere" is more descriptive of an example link, rather than a
button that triggers in-page functionality. Also makes it consistent
with all other examples in this page

also, stray "blocks" in the "Sample output", which doesn't seem to
belong there.

...as per the text immediately preceding this example

use `<code>` (as it's still semantically code that we're marking up) and
make the example actually multiple lines...

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Use the structure as documented in
http://v4-alpha.getbootstrap.com/components/navbar/#collapsible-content
- wrapping both the toggle and the navigation in a `<nav>`, using a
`<ul>` for the navigation items

It's unclear why the example currently has the content that's
shown/hidden BEFORE the toggle. Generally, this is not a recommended
content order, as after toggling, the newly shown content precedes the
toggle and requires keyboard users, and particularly assistive
technology users, to then navigate in reverse to reach it.

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Based on v3 http://getbootstrap.com/css/#forms-help-text, adapted to
make examples more "real life"

Use new `.breadcrumb-item` class instead of child selectors and `li` tag selectors,
so as to no longer require the usage of `<ol>`-based markup.

Rename variables to follow naming conventions:
* $breadcrumb-padding-vertical => $breadcrumb-padding-y
* $breadcrumb-padding-horizontal => $breadcrumb-padding-x

Introduce new variable: $breadcrumb-item-padding

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