feat(devmanual): Document IMailer and inline attachments

Signed-off-by: Christoph Wurst <christoph@winzerhof-wurst.at>

developer_manual/digging_deeper/email.rst

@@ -0,0 +1,46 @@
+=====
+Email
+=====
+
+Nextcloud has a mailer component to send email from an admin-defined account.
+
+Basic usage
+-----------
+
+The mailer is hidden behind the ``\OCP\Mail\IMailer`` interface that can be :ref:`injected<dependency-injection>`:
+
+.. code-block:: php
+    :caption: lib/Service/MailService.php
+
+    <?php
+
+    use OCP\Mail\IMailer;
+
+    class MailService {
+        private IMailer $mailer;
+
+        public function __construct(IMailer $mailer) {
+            $this->mailer = $mailer;
+        }
+
+        public function notify(string $email): void {
+            $message = $this->mailer->createMessage();
+            $message->setTo([$email]);
+            $this->mailer->send($message);
+        }
+    }
+
+Inline attachments
+------------------
+
+Inline attachments can be appended to a message with ``IMessage::attachInline``:
+
+.. code-block:: php
+
+    /** @var IMessage $message */
+    $message->attachInline(
+        "this is a test", // Body
+        "test.txt",       // Name
+        "text/plain"      // Content type
+    );
+    $this->mailer->send($message);

developer_manual/digging_deeper/index.rst

@@ -10,6 +10,7 @@ Digging deeper
    debugging
    classloader
    continuous_integration
+   email
    flow
    http_client
    javascript-apis