Document header dependencies for source distributions

[saitakarcesme]

Document set_source(..., depends=[...]) for project-local headers and distinguish it from include_dirs. A header can be found when compiling in the working tree but be absent when a wheel is built from an sdist. The overview example now declares pi.h as a dependency and includes the project directory in the compiler search path.

Fixes #168, following the discussion's preference to document the existing setuptools keyword.

Validation: built a minimal CFFI project through sdist → wheel with the checked-out CFFI. Without depends, pi.h was absent from the tarball and compilation failed; with it, the wheel built successfully, installed, and its C function returned the expected result. Both cases used include_dirs=['.']. sphinx-build -W -b html doc/source doc/build/html and git diff --check pass.

Disclosure: documentation and build verification prepared using OpenAI Codex in an authorized automated contribution session.

[mattip]

Thanks @saitakarcesme