[docs] Remove 'network.json' mentions

Summary:
Resolves ENG-2727.

Removed docs references to network.json and updated the section to mention the COMM_NAT_DEV_HOSTNAME env var instead.

Test Plan: Ensured markdown is rendering correctly.

Reviewers: ashoat, O6 Docs, inka

Reviewed By: ashoat, O6 Docs, inka

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D6308