gh-1286 : Document how to add a bytecode specialization

[faizanoor3001]

Solves: #1286

Description

• Add a "How to Add a New Bytecode Specialization” section in the dev guide.
  • This guide explains how to add a new bytecode specialization, using
    CONTAINS_OP as an example.

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📚 Documentation preview 📚: https://cpython-devguide--1522.org.readthedocs.build/

[cpython-cla-bot]

All commit authors signed the Contributor License Agreement.

[Fidget-Spinner]

Sorry, could you please open a PR at CPython and document it here instead? https://github.com/python/cpython/blob/main/InternalDocs/interpreter.md

I'll close this PR, as it seems the new trend is to document internal docs in CPython itself not the devguide.