Migrate setup.py references to pyproject.toml.

[bdice]

Description

Fixes outdated references to setup.py in the docs.

Aligns with https://github.com/glotzerlab/signac/pull/878, https://github.com/glotzerlab/signac-flow/pull/704.

Checklist:

• [x] I am familiar with the Contributing Guidelines.
• [x] I agree with the terms of the Contributor Agreement.
• [x] My code follows the code style guideline of this project.

[mikemhenry]

LGTM! I like that you added installing from source without having to clone the repo