Fix documentation about galery

Galery can be fixed by using internal resolution.
Adding the lang as well allows to make the target URL lang-aware even if the image is just duplicated.

Example file has been fixed as well even if it might not be used.

docs/content/documentation/content/image-processing/index.md

@@ -153,7 +153,7 @@ picture gallery with the following shortcode named `gallery.html`:
 {% for asset in page.assets -%}
   {%- if asset is matching("[.](jpg|png)$") -%}
     {% set image = resize_image(path=asset, width=240, height=180) %}
-    <a href="{{ get_url(path=asset) }}" target="_blank">
+    <a href="{{ get_url(path='@' ~ asset, lang=lang) }}" target="_blank">
       <img src="{{ image.url }}" />
     </a>
   {%- endif %}

docs/templates/shortcodes/gallery.html

@@ -2,7 +2,7 @@
 {% for asset in page.assets -%}
   {%- if asset is matching("[.](jpg|png)$") -%}
     {% set image = resize_image(path=asset, width=240, height=180) %}
-    <a href="{{ get_url(path=asset) }}" target="_blank">
+    <a href="{{ get_url(path='@' ~ asset, lang=lang) }}" target="_blank">
       <img src="{{ image.url }}" />
     </a>
   {%- endif %}