[doc] Include contributing.md in Jekyll (#2859)

Fixes <https://github.com/FreshRSS/FreshRSS/issues/2856> (hopefully).

docs/_config.yml

@@ -4,3 +4,5 @@ description: Documentation center
 
 logo: /img/FreshRSS-logo.png
 show_downloads: true
+
+include: contributing.md