docs: add clear project integration guide for pipeworks_ipc

[aa-parky]

Summary

• add a dedicated Sphinx page docs/source/project_integration.rst focused on real project adoption
• document how to replace copied IPC hashing code with the shared pipeworks_ipc package
• include a practical integration helper pattern, metadata persistence guidance, migration checklist, and deterministic test template
• wire the new page into docs navigation via docs/source/index.rst
• add a cross-link in docs/source/usage.rst to direct users to the integration guide

Validation

• make -C docs clean html
• pre-commit run --files docs/source/index.rst docs/source/usage.rst docs/source/project_integration.rst

Goal

This makes the docs explicit about why and how to consume pipeworks_ipc in downstream projects so IPC logic is reused instead of re-implemented.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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