docs: create a new "advanced" book

docs/guide/src/concepts/errorhandling.md

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+# Error handling
+
+
+Astute observers might have noticed that `Element` is actually a type alias for `Option<VNode>`. You don't need to know what a `VNode` is, but it's important to recognize that we could actually return nothing at all:
+
+```rust
+fn App((cx, props): Component<()>) -> Element {
+    None
+}
+```
+