docs: add contributing guides

[marandaneto]

💡 Motivation and Context

The repo-level and posthog-rails package development instructions were split across README files.

This PR moves that contributor guidance into dedicated CONTRIBUTING.md files so both the root package and posthog-rails follow the same convention.

💚 How did you test it?

• Not run (docs-only changes)
• Checked the updated markdown links and file locations

📝 Checklist

• I reviewed the submitted code.
• I added tests to verify the changes.
• I updated the docs if needed.
• No breaking change or entry added to the changelog.

If releasing new changes

• Ran pnpm changeset to generate a changeset file
• Added the release label to the PR