[Docs] Update supported workflow information

Summary:
If this document is meant to be the entrypoint for developers, having
the supported workflow table the second item is very distracting; so move it
to the bottom where more relevant information is presented first.

Depends on D4967

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

Reviewers: ashoat, varun

Reviewed By: ashoat

Subscribers: tomek, atul

Differential Revision: https://phab.comm.dev/D4968