Add pylibcudf source annotations for stub generation

[vyasr]

Summary

• add source-level return annotations for pylibcudf APIs where Cython and stubgen-pyx can preserve the intended types
• document pylibcudf type-stub/source-annotation guidance in the developer guide
• keep this branch source/docs-only, without generated .pyi replacements or the stubgen pre-commit hook

Notes

This is split out from the generated-stubs work so it can be reviewed independently. The annotations intentionally avoid cpdef container returns involving extension-class element types because current stubgen-pyx output degrades those to object.

Validation

• pre-commit run --files ... for the touched pylibcudf source annotation files, run inside the cudf devcontainer
• pre-commit run --files docs/cudf/source/pylibcudf/developer_docs.md, run inside the cudf devcontainer

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