Add thankContributors option and changelog formatters docs

[theoephraim]

Summary

• Add thankContributors boolean option to the GitHub changelog formatter so "Thanks @user" messages can be disabled entirely
• Add dedicated changelog formatters docs page covering the default formatter, GitHub formatter with all options, and custom formatter examples
• Trim the changelog section in configuration.md to link to the new page
• Switch project config to use the GitHub formatter

Test plan

• Verify thankContributors: false suppresses thanks messages in generated changelogs
• Verify thankContributors: true (default) still includes thanks messages
• Review docs page for accuracy and completeness

🤖 Generated with Claude Code

[github-actions]

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a bump file.

You can add a bump file by running:

bunx bumpy add

Or click here to add a bump file directly on GitHub.

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This comment is maintained by bumpy.