Use "eu_categories" frontmatter attribute

Jekyll treats names like "category" and "categories" differently, so I chose to
name it "eu_categories" in order to avoid inheriting the special treatment, and
have full control of what a category means and how it behaves.

The _includes/category.html was changed to reflect that, with an extra coupled
change that makes it not specific to articles, but any collection type.

1 files changed, 1 insertions, 1 deletions

diff --git a/categories.en.md b/categories.en.md
index 49a5331..c7cda8f 100644
--- a/categories.en.md
+++ b/categories.en.md
@@ -10,4 +10,4 @@ ref: categories
 
 ---
 
-{% include categories.html %}
+{% include categories.html kind="articles" %}