Replace container images to packages from cpython-devcontainers #1441
Description
Describe the bug
When I wrote the devguide for adding extensions modules, I assumed that we'll continue using Ubuntu public images: https://devguide.python.org/developer-workflow/extension-modules/#make-regen-configure-and-missing-permissions.
However, this is no more the case and we are using a GHCR package (see python/cpython#124612). I don't think it was actually necessary to show how to do a manual configuration. I think using podman is better than using docker so I'll rather recommend the former. At the same time, I'll also be able to remove the small Sphinx extension that I wrote to simplify upgrading the image.
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