Docs(Scrollspy): minor changes to be aligned with new version of javascript (#36260)

Docs(Scrollspy): minor changes to be aligned with new version of javascript  (#36260)

* Docs(Scrollspy): minor changes to be aligned with new version of javascript code

* first example: add proper root-margin
* 'nested nav' example: add more content to sections & enable smooth-scroll

* fix typo

* Changes after feedback

* Update examples and docs to streamline some things and improve how it works section

Co-authored-by: Mark Otto <markdotto@gmail.com>