[Docs] Update nix_dev_env.md to reference relevant docs

Summary:
Add ToC to nix_dev_env to reference the required steps to setup
a development environment.

Depends on D4967

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

Reviewers: abosh, varun, ashoat

Reviewed By: abosh, ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D4978