Add a test package and docs for the "C/C++ lib with optional Python bindings" use case

This follows up on the discussion at https://github.com/mesonbuild/meson-python/pull/728#discussion_r2069077299. The use case is multi-language projects, like the Arrow projects, where the root of the repo contains a core C/C++ library and bindings for Python (and perhaps other languages) are present in sub-directories. This is a fairly common layout:

```
meson.build
src/

meson.build
python/

pyproject.toml
meson.build
```

The challenge here is to get the top-level library built and linked against. The idea in the linked discussion is to add:
```
python/
dist_script.py # replaces symlink with a copy for sdist generation
subprojects/

```

We've seen other people struggle with this puzzle when trying to adopt `meson-python`, e.g. in https://github.com/mesonbuild/meson-python/discussions/674. It would be good to have a test package for this project structure, and a bit of documentation to point out how this can be handled.

Start with the linked pull-request discussion and the project layout described in the issue, then review the existing test packages and documentation for related use cases. Add a test package covering a C/C++ library with Python bindings in a subdirectory, plus guidance for the layout and sdist handling. Done means the package tests build and link the top-level library and the documented workflow is clear.

Written by the indexing model from the issue text.