Development container

Set up a minimal development container for the first item below. Other enhancements might be pulled to separate issues.

* [x] edit a markdown tutorial in the GitHub editor, and preview the changes before committing
* [x] edit Python code in the GitHub editor, and run the unit tests before committing
* [x] edit Python code in the GitHub editor, and see the changes in a browser tutorial before committing
* [ ] edit Python code or Java code in the GitHub editor, and download a plugin jar to test in PyCharm, before committing
* [ ] edit Python code or Java code in the GitHub editor or in a local docker container, and use IDEA to step through the code

JetBrains isn't yet listed on [containers.dev], but their [Gateway application] sounds similar.

When we launch a Jekyll server, it sounds like GitHub will [open a public port] for it.

There might be some relevant details in [Sarah Kaiser's slides], when they're published.

[containers.dev]: https://containers.dev/supporting
[Gateway application]: https://www.jetbrains.com/remote-development/gateway/
[open a public port]: https://stackoverflow.com/a/63911342/4794
[Sarah Kaiser's slides]: https://2023.pycascades.com/program/talks/eternal-sunshine-of-the-spotless-development-environment/

The issue does not name a repository file, test, or entry point, and its checklist mixes completed and possible future workflows. First clarify which unchecked workflow the minimal container must support, then review the containers.dev, JetBrains Gateway, public-port, and Sarah Kaiser references. Done means the agreed first workflow works in the development container.

Written by the indexing model from the issue text.