[landing] introduce feature modal component

Summary:
This diff introduces the feature modal that will pop up whenever a user clicks on a feature comparison card. This diff just introduces the structure and style of the modal and subsequent diffs will put everything together. The ui for the modal did deviate a bit from what is on the Figma design; however, given the structure of the competitor data copy, I thought it was better to have some separation between the texts and to show the company logos, instead of just one giant paragraph (cc @ted). Here is a screenshot of the figma for reference:

Depends on D8537

Test Plan:
Please watch the demo video to see how the feature modal looks and behaves:

General behavior:

Responsiveness:

Mobile:

Tablet:

Reviewers: atul, kamil

Reviewed By: atul

Subscribers: kuba, ted, ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D8025