chore(docs): add directive to pad heading anchors

Closes #2070
Closes #9360
The idea is to support having ngOffset evaluate to something under some condition, or evaluate to undefined or null if the condition is not met and let the default headingOffset be used.

E.g. having the following HTML:

    <h1 id="heading1" ng-offset="{{weAreOnSmallScreen() ? '0px' : ''}}">Heading 1</h1>

we could account for the following scenario (based on how the docs app is designed):
if we are on a small screen, the header's positioning becomes static (not fixed), so we don't need an offset. So, if `weAreOnSmallScreen()` evaluates to true, use the value of ngOffset (which is `'0px'`).
If we are on a large screen, the header is fixed and we should use the default offset of `headingOffset.value`. `weAreOnSmallScreen()` evaluates to true, `attrs.ngOffset` evaluates to `''` (empty string) and the `updateStyle()` function understands that it should use the default offset.

This is indeed an implementation choice (i.e. nothing inherenty wrong if we don't support it) and the added flexibility might not be worth the trouble. Up to you :)