Improve subrule documentation

### Page link:

https://bazel.build/rules/lib/builtins/Subrule

### Problem description (include actual vs expected text, if applicable):

Bazel 8.0 [flipped](https://github.com/bazelbuild/bazel/commit/80d8e4253332961b703f67c0b33e6e6f803dd5b7) the `--experimental_rule_extension_api` flag to true. But this wasn't mentioned in the [release notes](https://github.com/bazelbuild/bazel/releases/tag/8.0.0), and the official [documentation](https://bazel.build/rules/lib/builtins/Subrule) for subrules just points to the design doc.

I've found subrules to be a valuable way to structure my Starlark code, and I use dozens of them in my Bazel repo. I'd like subrules to be better supported/documented across the Bazel ecosystem. This includes documenting the subrule API, but also providing APIs for documenting subrules themselves. (Currently, starlark_doc_extract ignores subrules.)

### Where do you see this issue? (include link to specific section of the page, if applicable)

_No response_

### Any other information you'd like to share?

_No response_