ANCHOR SCROLL: docs app: adjust scroll offset to account for header

[IgorMinar]

go to https://docs.angularjs.org/api/ng/service/$animate#removeClass and you'll see that the scroll position is not right because it doesn't account for the app header.

we could use the solution described here to fix this: #2070 (comment)

[IgorMinar]

@petebacondarwin can you take a look at this please?

if the proposed solution works we should close #2070 and point to the fix as an example of how people can use it in their apps too.

[IgorMinar]

if this can't be done within rc.5 time frame please let me know

[gkalpak]

@IgorMinar , @petebacondarwin:

I created a POC PR for adding a configurable scroll offset to $anchorScroll: #9371
It is still a WIP (so no docs and no tests yet).

I also configured the docs app:
If the header's positioning is 'fixed' (in large screens), the offset will successfully account for the header's (variable) height. If the positioning is 'static' (in smaller screens) then the offset returned is 0.

It seems to work nicely in every situation (all screen sizes and all elements (<li>s included).

WDYT ?

_{Pleae, don't scream about how I get a hold on the header element. It's a super-temporary, ugly, POCy hack :)}