[landing] add all section components to AppLanding

Summary:
Now that we have built all the components for each section. This diff just simply adds them to the AppLanding component.

Depends on D8080

Test Plan:
Please see the demo video below:

Reviewers: atul, kamil

Reviewed By: kamil

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8081