Add pico-theme.yml and update to API version 3

See https://github.com/picocms/Pico/commit/b27b4f388a34210a94ab59fc12faececb5435d4c for more info

index.twig

@@ -47,7 +47,7 @@
 
     <div id="main">
         <div class="container">
-            {{ content }}
+            {{ content|raw }}
         </div>
     </div>
 

pico-theme.yml

+api_version: 3
+
+meta:
+    Social: social
+
+twig_config:
+    autoescape: html
+    strict_variables: false
+    charset: utf-8
+
+widescreen: false               # Use more horicontal space (i.e. make the site container wider)