docs: update transports.md

pino-datadog-transport is no longer maintained

Ref: https://github.com/theogravity/datadog-transports/tree/main/packages/pino-datadog-transport

I've created a separate package to maintain it forward, with extra features like batching and proper AWS Lambda support

Changes

Update transports.md to include the new link

Testing

✅ All existing tests pass
✅ TypeScript configuration is now syntactically correct
✅ Linting passes
✅ No breaking changes

Type of Change

• Bug fix (non-breaking change which fixes an issue)
• New feature
• Breaking change
• Documentation update

Checklist

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas (N/A for this change)
• I have made corresponding changes to the documentation (N/A for this change)
• My changes generate no new warnings
• I have added tests that prove my fix is effective (N/A - syntax fix)
• New and existing unit tests pass locally with my changes