v4: Docs Sass linting

Docs: accessibility fix-up of collapsible content navbar, change site-wide main navbar

remove `<div class="bd-sidebar">`

Docs: add note and advice about .disabled and keyboard focus

Remove .dl-horizontal; it's redundant to the grid system

Docs: tweaks to content code

Add tether as bower dependency

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

[ci skip]

[ci skip]

[ci skip]

Refs #18728
[ci skip]

Fixed NuGet link and bugs in MyGet script

Docs: update browser support info

Add pointer-events:none to disabled pagination/pager links

Add max-width to file browser; fixes #18651

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.

as currently this was missed out (compared to disabled `a.btn`)

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

Docs: add accessibility callout for tooltips

Fix typo in docs/getting-started/introduction.md

Remove 'including'.

Docs: change "Button" to "Go somewhere"