Fix link to schleuder's issue tracker from docs/...

By default, the participate partial now links to schleuder's issue
tracker.

_includes/participate.md

@@ -2,6 +2,6 @@
 
 <div class="participate">
 
-To **participate** in the development use the [issue tracker](https://0xacab.org/schleuder/{{ include.project }}/issues). Please take note of our [Code of Conduct]({{ "CODE_OF_CONDUCT.html" | absolute_url }}).
+To **participate** in the development use the [issue tracker](https://0xacab.org/schleuder/{{ include.project | default: "schleuder" }}/issues). Please take note of our [Code of Conduct]({{ "CODE_OF_CONDUCT.html" | absolute_url }}).
 
 </div>