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

[rgommers]

This follows up on the discussion at #728 (comment). 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/
    <some_c_sources>
    meson.build
python/
    <some_python_sources>
    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/
        <symlink with content `../..`>

We've seen other people struggle with this puzzle when trying to adopt meson-python, e.g. in #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.