fix: open section nav should close on scroll

[ramboz]

If the user scrolls for more than 50px from the opening point, we will close the section navigation menu. This allows for some minimal margin in case of unintentional scrolls, and for accessibility.

Fix #140

Test URLs:

• Before: https://main--delta--hlxsites.hlx.page/us/en/skymiles/how-to-earn-miles/overview
• After: https://issue-140--delta--hlxsites.hlx.page/us/en/skymiles/how-to-earn-miles/overview

[aem-code-sync[bot]]

Hello, I'm Franklin Bot and I will run some test suites that validate the page speed. In case there are problems, just click the checkbox below to rerun the respective action.

• [ ] Re-run PSI Checks

[aem-code-sync[bot]]

Page	Score	PSI Audit	Google
/us/en/skymiles/how-to-earn-miles/overview