docs: more docs

docs/main-concepts/05-context-api.md

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+# Context API
+
+```rust
+// Create contexts available to children
+// Only one context can be associated with any given component
+// This is known as "exposed state". Children can access this context,
+// but will not be automatically subscribed.
+fn ContextCreate(cx: &mut Context<()>) -> VNode {
+    let context = cx.set_context(|| CustomContext::new());
+    html! { <> {cx.children()} </> }
+}
+
+fn ContextRead(cx: &mut Context<()>) -> VNode {
+    // Panics if context is not available
+    let some_cx = cx.get_context::<CustomContext>();
+    let text = some_cx.select("some_selector");
+    html! { <div> "{text}" </div> }
+}
+
+fn Subscription(cx: &mut Context<()>) -> VNode {
+    // Open a "port" on the component for actions to trigger a re-evaluation
+    let subscription = cx.new_subscription();
+
+    // A looping timer - the effect is re-called on every re-evaluation
+    use_async_effect(cx, move || async {
+        timer::new(2000).await;
+        subscription.call();
+    }, None);
+
+    // A one-shot timer, the deps don't change so the effect only happens once
+    use_async_effect_deps(cx, move || async {
+        timer::new(2000).await;
+        subscription.call();
+    }, ());
+}
+
+// Mix subscriptions and context to make a simple Redux
+fn use_global_state<T: UserContextTrait>(cx: &mut Context<()>) -> T {
+    let some_cx = cx.get_context::<T>();
+    let component_subscription = cx.new_subscription();
+    some_cx.subscribe_component(component_subscription);
+    some_cx
+}
+
+```