Contributing guidelines now visible in repository tab and sidebar

[tuves]

What's changed

New contributors often miss detailed contributing guidelines because these files are hard to find within repositories. Their poor discoverability creates friction for newcomers, limiting onboarding and community growth despite maintainers’ feedback. So, we’ve surfaced these guidelines in two key places:

1. Repository overview - A Contributing tab has been added to the repository overview (next to README and Code of conduct).
2. Repository sidebar - A Contributing link has been added to the repository sidebar.

Both will appear whenever a repo has a CONTRIBUTING.md file, and you’ll no longer have to search for the file. This update helps new contributors get started faster and streamlines onboarding for open source projects.