Offset for jump links on mobile

[AndreeaCristinaRadacina]

What problem does this address?

A user reported that the anchor links won't work correctly on mobile devices.
I also tested, and this issue happens depending on the height of the sections.

Live behavior: https://visa.digiso.co.uk/#uk-visa-assistance

Ref: https://secure.helpscout.net/conversation/2432188142/394276?folderId=212385

What is your proposed solution?

Add an offset option somewhere inside the header builder, maybe.

Seems to be somehow similar to this - #3818, just that this user doesn't utilize a sticky header.

Will this feature require documentation? (Optional)

Yes, it requires documentation.