chore(docs-app): fix vertical scrolling offset after recent re-design
chore(docs-app): fix vertical scrolling offset after recent re-design

Previously, the `yOffset` pointed to the `<header>` element, which had a height
of 0 (since all its children have fixed positions). This caused scrolled items
to have 0 vertical offset, essentially hiding the top part behind the static
`<nav>` items.
This commit fixes the vertical scrolling offset, by setting `yOffset` to the
last (lowest) `<nav>` item.

Closes #15945

angularbrasil/angular.js,marcin-wosinek/angular.js,angular/angular.js,hnccho/angular.js,lgalfaso/angular.js,ze-pequeno/angular.js,rbevers/angular.js,ze-pequeno/angular.js,mgol/angular.js,jbedard/angular.js,nsina/angular.js,eistrati/angular.js,gkalpak/angular.js,reneploetz/angular.js,mgol/angular.js,ze-pequeno/angular.js,marcin-wosinek/angular.js,dmuellner/angular.js,Narretz/angular.js,nsina/angular.js,angular/angular.js,dmuellner/angular.js,pbr1111/angular.js,hnccho/angular.js,rbevers/angular.js,hnccho/angular.js,Narretz/angular.js,mosoft521/angular.js,nsina/angular.js,arunkannan/angular.js,jbedard/angular.js,Narretz/angular.js,lgalfaso/angular.js,angular/angular.js,gkalpak/angular.js,mosoft521/angular.js,sercaneraslan/angular.js,rbevers/angular.js,lgalfaso/angular.js,dmuellner/angular.js,mgol/angular.js,ze-pequeno/angular.js,gkalpak/angular.js,mgol/angular.js,lgalfaso/angular.js,mosoft521/angular.js,sercaneraslan/angular.js,eistrati/angular.js,sercaneraslan/angular.js,dmuellner/angular.js,jbedard/angular.js,marcin-wosinek/angular.js,reneploetz/angular.js,mosoft521/angular.js,pbr1111/angular.js,reneploetz/angular.js,rbevers/angular.js,jbedard/angular.js,reneploetz/angular.js,pbr1111/angular.js,sercaneraslan/angular.js,arunkannan/angular.js,angularbrasil/angular.js,angularbrasil/angular.js,angularbrasil/angular.js,arunkannan/angular.js,arunkannan/angular.js,eistrati/angular.js,hnccho/angular.js,nsina/angular.js,eistrati/angular.js,angular/angular.js,gkalpak/angular.js,pbr1111/angular.js,Narretz/angular.js,marcin-wosinek/angular.js