Add docs for the new auto option in image tags

content/docs/2_reference/1_text/0_kirbytags/0_image/reference-kirbytag.txt

@@ -126,6 +126,25 @@ You can customize the defaults of this tag's attributes via the (link: docs/refe
 ```kirbytext
 (\image: myawesomepicture.jpg width: 500 height: 300)
 ```
+<since v="4.4.0">
+Width and height can also be set to auto, which will automatically be replaced by the actual image dimensions.
+
+```kirbytext
+(\image: cat.jpg width: auto height: auto)
+```
+
+This behaviour can also be set as global default for all image kirbytags in your config file.
+
+```php
+// config.php
+'kirbytext' => [
+	'image' => [
+		'width' => 'auto',
+		'height' => 'auto',
+	]
+];
+```
+</since>
 
 <since v="4.0.0">
 ### Image with `srcset` with absolut sizes