feat: add smtp adapter (#279)

lib/notification/adapter/smtp.md

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+### SMTP Adapter
+
+Send notifications through any SMTP server using [Nodemailer](https://nodemailer.com/).
+This works with Gmail, Outlook, self-hosted mail servers, or any provider that supports SMTP.
+
+Setup:
+
+- Provide the SMTP host and port of your mail server.
+- For **SSL/TLS** (port 465), set Secure to `true`.
+- For **STARTTLS** (port 587), leave Secure empty or set it to `false`.
+- Enter the username and password for authentication. For Gmail, use an [App Password](https://support.google.com/accounts/answer/185833).
+- Set the sender email address (must be allowed by your SMTP server).
+
+Multiple recipients:
+
+- Separate email addresses with commas (e.g., `some@email.com`, `someOther@email.com`).
+
+Common SMTP settings:
+
+- **Gmail** — `smtp.gmail.com`, port 587, secure: false
+- **Outlook** — `smtp.office365.com`, port 587, secure: false
+- **Yahoo** — `smtp.mail.yahoo.com`, port 465, secure: true