[Nix Docs] Copy phabricator account creation section

Summary:
Was able to find the related header for all other sections, only
initial Phabricator account creation was missing.

This is just a straight copy of https://github.com/CommE2E/comm/blob/abe832b337bc320952e6bfd029e623736b6075c9/docs/dev_environment.md#phabricator. Edits to the section will be in the following diff to make review easier.

https://linear.app/comm/issue/ENG-3001

Test Plan: N/A

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: varun, tomek, atul

Differential Revision: https://phab.comm.dev/D6725