[docs] Sort things better between mobile/web Nix workflow docs

Summary:

• We presumably don't need lines referring to mobile workflows in the web workflows doc
• Both docs need to cover keyserver and Flow stuff, since it's required for both workflows
• Mobile workflows need to cover Redux and Redux dev tools as well

Test Plan: Read through each and made sure the necessary steps were included, and irrelevant steps were excluded

Reviewers: jon, bartek

Reviewed By: jon

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D5980