[Nix Docs] Mention yarn cleaninstall and git clone

Summary:
Ensure that developers have the repository pulled down and that
yarn cleaninstall is ran as part of a development workflow.

https://linear.app/comm/issue/ENG-2242

Test Plan: N/A

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek, atul, abosh

Differential Revision: https://phab.comm.dev/D5652