[Docs] Point to Nix docs as default developer documentation

Summary:
Move references to developer documentation to point to Nix docs.

https://linear.app/comm/issue/ENG-3000

Depends on D6725

Test Plan: Rendered form https://github.com/CommE2E/comm/blob/jonringer/nix-docs-supercede/docs/dev_environment.md

Reviewers: ashoat, varun

Reviewed By: ashoat, varun

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6727