Setup error-codes/index

Author: WojciechMazur

docs/_docs/reference/error-codes/error-codes.md

@@ -1,3 +1,27 @@
 ---
+layout: index
 title: Error codes
----
+---
+
+The Scala 3 compiler assigns a unique error code to each type of error it can emit.
+These codes appear in compiler output in the format `[E001]`, `[E007]`, etc.
+
+This section documents each error code with:
+
+- **Description** — what the error means and when it occurs
+- **Example** — code that triggers the error
+- **Error output** — the actual compiler message you'll see
+- **Solution** — how to fix the problem
+
+## Using error codes
+
+When you encounter an error, you can use the `-explain` flag to get more detailed information:
+
+```bash
+scala compile -explain MyFile.scala
+```
+
+You can also look up any error code directly in this documentation by its number (e.g., E007 for type mismatch errors).
+
+Browse the full list in the sidebar to find documentation for any specific error code.
+