Add content to How To summary page (zio#2256)

* Add content to How To summary page

* Add some description

docs/howto/index.md

@@ -1,6 +1,12 @@
 ---
 id: howto_index
-title:  "Summary"
+title: "Summary"
 ---
 
-Coming soon
+Here are a few guides for common patterns with ZIO:
+
+- **[Use the module pattern](use_module_pattern.md)**: how to structure larger ZIO programs with the help of **ZIO environment**.
+- **[Test effects](test_effects.md)**: how to use **ZIO Test** to test effectual programs seamlessly.
+- **[Mock services](mock_services.md)**: how to test interactions between services with **mocks**.
+- **[Handle errors](handle_errors.md)**: how to deal with **errors** in ZIO (declared errors vs unforeseen defects).
+- **[Access system information](system.md)**: how to access **environment variables** and other **system properties** with ZIO.