[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