add support for redirects via metadata extension

docs/extensions/metadata.md

@@ -48,6 +48,21 @@ see on the current page when you scroll to the top. It's as simple as:
 ``` markdown
 hero: Metadata enables hero teaser texts
 ```
+### Redirects
+
+It's sometimes necessary to move documents around in the navigation tree and
+redirect user from the old URL to the new one. The `redirect:` meta-tag allows
+to create a redirection from the current document to the address specified in
+the tag.
+
+For instance, if your document contains:
+
+``` markdown
+redirect: /new/url
+
+```
+accessing that document's URL will automatically redirect to `/new/url`.
+
 
 ### Linking sources
 

src/main.html

@@ -20,4 +20,14 @@
   IN THE SOFTWARE.
 -->
 
+{% if page.meta.redirect %}
+<script>
+    location.href = '{{ page.meta.redirect }}')
+</script>
+<meta http-equiv="refresh" content="0; url={{ page.meta.redirect }}"/>
+<link rel="canonical" href="{{ page.meta.redirect }} " />
+{% else %}
+
 {% extends "base.html" %}
+
+{% endif %}