Update doctest glob for markdown files

I think we need to update [this](https://github.com/DiamondLightSource/python-copier-template/blob/main/template/pyproject.toml.jinja#L63) to support `.md`.

Open template/pyproject.toml.jinja at the linked line and inspect the doctest glob configuration. Run the repository's existing checks or tests, then verify that the generated configuration includes Markdown files and that doctests in .md files are discovered.

Vom Indexierungsmodell aus dem Issue-Text verfasst.