[landing] introduce signal competitor data

Summary:
This diff introduces the competitor data for signal which will be used in the competitor comparison. Right now I only have copy for Signal, but there is a Linear task to track the other competitors. As the tasks get completed, I will update the document accordingly

Depends on D8015

Test Plan: Read through carefully and will be further tested in subsequent diffs

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7904