Add instructions for cloning the repository using GitHub CLI

[CasP0]

Add instructions for cloning the repository using GitHub CLI.

• README.md: Add a section on how to clone the repository using gh repo clone python/cpython. Mention the prerequisites for using GitHub CLI.
• Doc/README.rst: Add instructions on cloning the repository using gh repo clone python/cpython. Include a note on installing GitHub CLI if not already installed.
• Doc/using/unix.rst: Add a section on cloning the repository using gh repo clone python/cpython. Mention the prerequisites for using GitHub CLI.

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📚 Documentation preview 📚: https://cpython-previews--128471.org.readthedocs.build/

[ghost]

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[zanieb]

Hi. Can you describe the motivation for this change?

This seems like a better fit for the devguide, e.g., at https://devguide.python.org/getting-started/setup-building/#get-the-source-code

This also adds a new README.md instead of modifying the existing README.rst.

[rruuaanng]

I believe that modifying the README.md usually concerns the project's appearance, and it requires input from the core team for suggestions. cc @erlend-aasland (He has provided me with significant assistance in documentation in the past. And he is from the core team.)
(By the way, we should sign CLA to protect your rights regarding this submission.)

[skirpichev]

Thank you for your contribution, but I think this should be closed. README points to the Developer Guide. If you wish to reorganize documentation - I think it's severe enough to open first an issue or (better) a discussion thread in https://discuss.python.org/

Else - please open pr against https://github.com/python/devguide.