* Make vf-navigation--on-this-page more dynamic

- allows for multiple on this page navigators
- activates the indicator only when the content is in focus
- is a breaking change! (requires a `data-vf-js-navigation-on-this-page-link` on the navigation links)

I think this is the *better* approach, but it's also disruptive as it has a breaking change.

* Update README.md

* Update vf-navigation.njk

* Update CHANGELOG.md

* Make onscreen math more reliable