add back Animation Add-Ons to the nav

[swyxio]

currently, there is an Animation Add-Ons page, however, this is not included in the nav and not indexed by algolia. this means that you basically only find two sentences on animation when you search "animation" on reactjs.org. related issue and brief discussion:

#1099 (comment)

While reactjs.org does deserve better animation docs longer term (addressed in that issue), I propose adding this doc back as a short term measure since a discrepancy between algolia and the google search index is definitely confusing.

[ryota-murakami]

I agree with you.
Firstly I thought the Animation Add-Ons page was outdated because that is not contained in nav.

[gaearon]

While I agree we should have a better guide, I don't think the "Animation Add-on" itself is a useful thing to have on our website. It's not maintained by us — it lives here now: https://github.com/reactjs/react-transition-group

[ryota-murakami]

Oh, I did't know.

It's not maintained by us

I think that should consider how to handle a Animation Add-Ons at #1099