Add a guide to deprecating features

[Lincoln-developer]

This pull request introduces a new section in the Developer Guide to document the practical steps for deprecating features in CPython. It complements the policy guidelines outlined in PEP 387 by providing a detailed, actionable workflow for contributors

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📚 Documentation preview 📚: https://cpython-devguide--1469.org.readthedocs.build/developer-workflow/cpython-deprecation-workflow/

[python-cla-bot[bot]]

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