[Docs] Organize comprehensive nix documentation

Summary:
Large addition of documentation specific to nix.

The content has been copied and pasted from the the existing
dev_environment.md with exception of initial headers.

Depends on D4966

Test Plan:
View rendered markdown:
https://github.com/CommE2E/comm/blob/jonringer/nix-dev-docs/docs/nix_dev_env.md

Reviewers: ashoat, abosh, varun

Reviewed By: abosh

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D4967