Add smooth scroll

[federicobucchi]

Bug/issue #985

Summary

Added CSS in html to enable the smooth scroll. Disable the smooth scroll if the user has a setting that enables prefers-reduced-motion

Dependencies

None

Testing

All done

Steps:

• Navigate to a page with multiple headings / anchors
• Click on an anchor
• User should see a smooth scroll to the anchor

Checklist

Make sure you check off the following items. If they cannot be completed, provide a reason.

• Added tests
• Ran npm test, and it succeeded
• Updated documentation if necessary

Tests are not needed for this and documentation shouldn't be updated for this.

[heckj]

@swift-ci please test

[mportiz08]

Sorry for not checking this PR out until now—I think I just missed it!

I don't see any issue with this PR in terms of the goal, but I think there are issues with the existing application that are preventing this from working as smoothly as it should 😅

When testing this locally with the PR changes, I'm sometimes seeing a regression where clicking a subsection further down on the page only partially scrolls to the right place whereas it always scrolls to the right place without these changes.

example.mov

If I had to guess, this is likely some sort of race condition coming from logic we have related to vue-router and scroll events and not any issue with the CSS here. However, we would want to address that before taking these changes.