docs: add missing heading

Add an H1 heading to encrypted_images.md.

Signed-off-by: Ruth Fuchss <ruth.fuchss@nordicsemi.no>
diff --git a/docs/encrypted_images.md b/docs/encrypted_images.md
index 005e51b..c7e35f8 100644
--- a/docs/encrypted_images.md
+++ b/docs/encrypted_images.md
@@ -19,6 +19,8 @@
 #
 -->
 
+# Encrypted images
+
 ## Rationale
 
 To provide confidentiality of image data while in transport to the