Create configuration-specific doc

[kaylareopelle]

The .NET agent has a document with configuration values available to the agent. I think something similar would be helpful for Ruby users.

This issue might belong in open-telemetry/opentelemetry.io.

[github-actions]

👋 This issue has been marked as stale because it has been open with no activity. You can: comment on the issue or remove the stale label to hold stale off for a while, add the keep label to hold stale off permanently, or do nothing. If you do nothing this issue will be closed eventually by the stale bot.