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

[GeoSot]

Changes:

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

closes #36198

@louismaximepiton please, you may take a look and feedback me

[louismaximepiton]

Thanks a lot for these corrections, they look good to me,

However :

• when a spy is clicked on the third example, the component moves under the header
• there are still some issues with the scroll on the nested nav

[GeoSot]

Thank you a lot for your help 😄

when a spy is clicked on the third example, the component moves under the header

It was the default mode, ( I didn't like it either ) but I've changed it

there are still some issues with the scroll on the nested nav

I am aware of them, but as before, the same problem comes in front. As javascript itself is not so much clever to decide which visible point has to be active (plus the observer restrictions), all functionality sums up to developer thoughts and skills (in this situation, mine's 😋 )

So it would be great if anyone else take a look and come up with a more precise and of course minimalist solution. Ps: Passing the js tests is one more hidden challenge, in this specific component

[mdo]

@GeoSot I pushed a bunch of updates here to the docs to clean up some of the new copy, but also rewrite a lot of content to better explain how it all works up front. Should be good to go!