docs: even more docs

docs/guide/src/interactivity/lifecycles.md

@@ -1,3 +1,11 @@
-# Lifecycle, updates, and effects
+# Effects
+
+In theory, your UI code should be side-effect free. Whenever a component renders, all of its state should be prepared ahead of time. In reality, we often need to perform some sort of side effect. Possible effects include:
+
+- Logging some data
+- Pre-fetching some data
+- Attaching code to native elements
+- Cleaning up
+
+This section is organized under interactivity because effects can be important to add things like transitions, videos, and other important media.
 
-> This section is currently under construction! 🏗