add mailing domain configuration documentation

MAILING_DOMAIN.md

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+# Mailing Domain Configuration
+
+This feature allows you to configure a separate domain specifically for email templates and links, while keeping your main `DOMAIN` setting for internal server access.
+
+## Configuration
+
+### Environment Variable
+Set the `MAILING_DOMAIN` environment variable:
+```bash
+MAILING_DOMAIN=https://public.example.com
+```
+
+### Docker Environment
+Add to your docker-compose.yml or docker run command:
+```yaml
+environment:
+  - MAILING_DOMAIN=https://public.example.com
+```
+
+Or with docker run:
+```bash
+docker run -e MAILING_DOMAIN=https://public.example.com vaultwarden/server
+```
+
+### Admin Panel
+The mailing domain can also be configured through the admin panel under SMTP Email Settings.
+
+## Use Cases
+
+1. **Internal vs Public Access**: Your Vaultwarden server runs on an internal domain (e.g., `http://vaultwarden.internal`) but you want emails to contain links to a public domain (e.g., `https://vault.company.com`).
+
+2. **Development vs Production**: Use different domains for email links in development and production environments.
+
+3. **Load Balancer/Proxy**: Your server runs behind a load balancer with a different internal address than the public-facing URL.
+
+## Behavior
+
+- If `MAILING_DOMAIN` is set, all email templates will use this domain for links and references
+- If `MAILING_DOMAIN` is not set, the system falls back to using the main `DOMAIN` setting
+- This affects all email types: invitations, password resets, 2FA emails, notifications, etc.
+
+## Example
+
+```bash
+# Main domain for server operations
+DOMAIN=http://vaultwarden.internal:8080
+
+# Public domain for email links
+MAILING_DOMAIN=https://vault.company.com
+```
+
+With this configuration:
+- The server operates on `http://vaultwarden.internal:8080`
+- All email links will point to `https://vault.company.com`