explore: mention the contribute page (MR 360)

It is otherwise not linked or discoverable from anywhere

EDIT by Oliver: it is mentioned, but only at the bottom left of the
page, this patch is useful :)

content/page/explore.md

 title: "Explore postmarketOS"
 ---
 
+Do you want to help make postmarketOS better? Check how to
+[contribute](/contribute)!
+
 ### 🧠 Knowledge
 
 * [**Blog**](/blog) —