epic: Docstrings / documentation approach that labels experimental features

[machow]

Currently, we are interested in pushing out features quickly. This is reflected especially in our docs, which simply push from main. This means that our docs can diverge from PyPI reality until we cut a release.

To be able to move quickly, but also set people's expectations, we should mark some features as experimental. It would be useful if people...

• Saw an experimental note in the docstring, IDE static hover over hint stuff, and web documentation
• Could click through on the web, to get some more information about what experimental means.
• The version in which something was added.
• (Optional), a badge for experimental a la r-lib/lifecycle.

[machow]

We just put a callout in the docstring of things like fmt_nanoplot, which feels okay for now.