[docs] Update Phabricator instructions in dev_environment.md

Summary:
These instructions are really confusing. They go too deep on things you really should not be doing, and don't explain things that you really need to understand.

This diff attempts to simplify the instructions by removing the unneeded parts, and also adds some things that everybody needs to know.

Linear task: ENG-427

Test Plan: I read through my edits carefully and made sure I don't have any stray single- or double-quote characters

Reviewers: varun, atul, palys-swm

Reviewed By: varun

Subscribers: Adrian, yayabosh

Differential Revision: https://phab.comm.dev/D4255