[Demo / docs] Add collapse to our heading sections

[andreascorti]

Sections can be long and it is not always easy to relate the content to the corresponding header.
Since now we have collapse component, we could use it to improve the display of our own documentation

[quentinderoubaix]

Note: there are links between sections (example link to onBeforeClose in ModalApi doc) and we should make sure they still work as expected.