feat: add initial draft of a new pattern 'walk the InnerSource talk'

[deveaud-m]

This pattern has been collaboratively written by the team behind the social coding platform at Siemens, summarizing key lessons learned from implementing Open Source methodologies across diverse and globally distributed development teams.

This pattern emerged from discussions with @spier during FOSS Backstage 2025 and subsequent conversations in issue #799. It addresses the "dogfooding" aspect of InnerSource initiatives - a crucial concept not yet covered by existing patterns. This PR documents how we've successfully implemented this approach at Siemens.

Looking forward to your feedback about this new pattern!

[welcome]

💖 Thanks for opening this pull request! 💖 The InnerSource Commons community really appreciates your time and effort to contribute to the project. Please make sure you have read our Contributing Guidelines.

If you are submitting a new pattern, the following things will help get your pull request across the finish line! 🏁

• Confirm that you have used our pattern template. Please remove any placeholder text and sections that your pattern did not need.
• We run a number of automated checks on your PR. Please review the output of those checks on the PR itself, and see if any issues got flagged that you can fix yourself.
• Make sure you have added your new pattern to the list of patterns in the main README.md. If you are unsure where to add your pattern, just let us know by commenting on your PR and we will help you.

This project has a small number of maintainers, volunteering their time to this project. So please be patient and we will get back to you as soon as we can. If we don't acknowledge this pull request after 7 days, feel free to chat to us about it in our Slack workspace.