Add ScrollSpy

[MaxLardenois]

Document our ScrollSpy

• maybe need some design ideas for the examples
• our implementation is based on ScrollSpy: deterministic active-section detection twbs/bootstrap#42557
• investigate if we keep or remove the smoothScroll option

⚠️ the parent selectors (to activate link's "parents" too) have been modified from the one in Bootstrap because our TOC do not use Bootstrap nav / nav-link. We will need to revisit this when implementing Scrollspy for our users.