Custom templates (#594)

* allow custom mail templates (#533)

* allow to override embedded frontend (#533)

docs/documentation/usage/mail-templates.md

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+WireGuard Portal sends emails when you share a configuration with a user. 
+By default, the application uses embedded templates. You can fully customize these emails by pointing the Portal 
+to a folder containing your own templates. If the folder is empty on startup, the default embedded templates 
+are written there to get you started.
+
+## Configuration
+
+To enable custom templates, set the `mail.templates_path` option in the application configuration file 
+or the `WG_PORTAL_MAIL_TEMPLATES_PATH` environment variable to a valid folder path.
+
+For example:
+
+```yaml
+mail:
+  # ... other mail options ...
+  # Path where custom email templates (.gotpl and .gohtml) are stored.
+  # If the directory is empty on startup, the default embedded templates
+  # will be written there so you can modify them.
+  # Leave empty to use embedded templates only.
+  templates_path: "/opt/wg-portal/mail-templates"
+```
+
+## Template files and names
+
+The system expects the following template names. Place files with these names in your `templates_path` to override the defaults.
+You do not need to override all templates, only the ones you want to customize should be present.
+
+- Text templates (`.gotpl`):
+  - `mail_with_link.gotpl`
+  - `mail_with_attachment.gotpl`
+- HTML templates (`.gohtml`):
+  - `mail_with_link.gohtml`
+  - `mail_with_attachment.gohtml`
+
+Both [text](https://pkg.go.dev/text/template) and [HTML templates](https://pkg.go.dev/html/template) are standard Go 
+templates and receive the following data fields, depending on the email type:
+
+- Common fields:
+  - `PortalUrl` (string) - external URL of the Portal
+  - `PortalName` (string) - site title/company name
+  - `User` (*domain.User) - the recipient user (may be partially populated when sending to a peer email)
+- Link email (`mail_with_link.*`):
+  - `Link` (string) - the download link
+- Attachment email (`mail_with_attachment.*`):
+  - `ConfigFileName` (string) - filename of the attached WireGuard config
+  - `QrcodePngName` (string) - CID content-id of the embedded QR code image
+
+Tip: You can inspect the embedded templates in the repository under [`internal/app/mail/tpl_files/`](https://github.com/h44z/wg-portal/tree/master/internal/app/mail/tpl_files) for reference. 
+When the directory at `templates_path` is empty, these files are copied to your folder so you can edit them in place.