Use pytest doctest to keep 'Get started' section up to date

[FynnBe]

Move 'get started' to bioimageio.core.init.py and maybe execute example commands with subprocess to have them validated with ptyest...

import subprocess
subprocess.run("bioimageio predict impartial-shrimp --example=True".split())

Maybe use https://docs.pytest.org/en/stable/how-to/doctest.html#doctest-namespace-fixture
to provide some run_example command to make it look like more closely to what is run in a terminal.
Maybe hide that line even with <!--- in the final rendered html.

<!---
>>>run_example("bioimageio predict impartial-shrimp --example=True") # --->
$ bioimageio predict impartial-shrimp --example=True