split add-ons into subsections

In preparation for documenting `classSet` add-on.
add docs on `classSet` add-on
update add-ons docs based on feedbacks

- Overview of add-ons for the add-ons page.
- Better title for `ReactLink`.
- Nicer code format for classSet example.
- Better explanation for `classSet`.
Merge branch 'chenglou-classSet'

closes #463