Auto-generate default settings.conf file

[Angeloth1]

• I have added the tests to cover my changes.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.

Summary

Adds auto-generation of the ~/.qiskit/settings.conf file. If the file does not already exist, the code safely creates the directory and the file, populating it with the default configurations commented out as a template.
Fixes #12539

Details and comments

• Implemented the auto-generation logic inside qiskit/user_config.py.
• Added the test_auto_generate_settings_conf unit test to verify directory/file creation and template content.
• Added a release note using reno.

AI tool used: Gemini 3.1 Pro

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• @Qiskit/terra-core

[Angeloth1]

Hey @Qiskit/terra-core . Just checking in, does the script look okay to you, or do I need to update it in any way? Thank you for your time.