docs(mailer): split docs and align deno config 📝

- Add `USAGE.md` with detailed SMTP usage, API reference, and troubleshooting
- Align `deno.json` compiler, lint, task, and publish settings to shared toolkit baseline
- Refactor `README.md` into concise landing page with badges and quick start
- Remove one extra blank line in SMTP message formatter

Author: NeaByteLab