docs: add missing rule to the docs

Author: incendial

website/docs/rules/index.mdx

@@ -260,6 +260,16 @@ Rules are grouped by category to help you understand their purpose. Each rule ha
   Enforces ordering for a class members.
 </RuleEntry>
 
+<RuleEntry
+  name="missing-test-assertion"
+  type="common"
+  severity="warning"
+  version="4.21.0"
+  hasConfig
+>
+  Warns that there is no assertion in the test.
+</RuleEntry>
+
 <RuleEntry
   name="newline-before-return"
   type="common"