[docs] Bring back to nix docs "Building and deploying the app" section

Summary:
As discussed in https://linear.app/comm/issue/ENG-2602/ip-port-in-dev-environmentmd the "Building and deploying the app" section is useful, so I'm bringing it back to the nix docs from which it was cut while moving the text from dev_environment.md
Here is this section in dev_environment.md : link

Test Plan: Viewed rendered markdown of nix_mobile_workflows.md

Reviewers: O6 Docs, ashoat

Reviewed By: O6 Docs, ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D6248