docs: create a partial tag helper to include shared markdown files

docs/source/_partial/then_should_difference.md

@@ -0,0 +1,7 @@
+all the differences in markdown
+
+***
+
+asdfkljasdfl
+
+## foo

docs/source/api/commands/and.md

@@ -3,6 +3,8 @@ title: and
 comments: true
 ---
 
+{% partial then_should_difference %}
+
 Make an assertion.
 
 {% note info %}