[landing] add rotation animation to keyserver faq icon

Summary:
This diff is the last part to the introduction of the keyserver faq section. This diff adds a rotation animation to the chevron icon so that when a faq item is opened then the chevron will transition from pointing down to pointing up

Depends on D8095

Test Plan:
Please see the demo video below:

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ted, ashoat, tomek

Differential Revision: https://phab.comm.dev/D8128