Use hatch everywhere?

[grst]

There are some commands where we suggest hatch in the docs, e.g.

hatch run docs:build  # defined in the table [tool.hatch.envs.docs]

however hatch support of this template seems half-baked.

For instance, CI could be running hatch as well, such that pyproject.toml becomes the single point of truth to define test commands or build matrices.

(Somewhat related #331)