Add blog link to sidenav (#12023)

Author: parlough

src/_data/sidenav.yml

@@ -132,6 +132,8 @@
     - title: Compatibility policy
       permalink: /release/compatibility-policy
     - divider
+    - title: Medium publication
+      permalink: https://medium.com/flutter
     - title: What's new in the docs
       permalink: /release/whats-new