[Nix Docs] Remove nix jargon in description

Summary:
Introductory paragraphs to Nix should avoid domain terms such as
shell hook or nativeBuildInputs.

https://linear.app/comm/issue/ENG-3007

Related to https://linear.app/comm/issue/ENG-2906

Depends on D6727

Test Plan: Rendered form https://github.com/CommE2E/comm/blob/jonringer/nix-docs-supercede/docs/nix_dev_env.md#how-nix-introduces-dependencies-to-a-development-environment

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6728