Add js web stuff to landing page + documentation

go/jsruntime/utils/accessors.ts

@@ -0,0 +1,14 @@
+// solid-js/html wrapProps auto-invokes zero-arg functions passed as component
+// props, so handlers must use a parameter. Readers must accept either the
+// unwrapped value or an accessor function.
+export type MaybeAccessor<T> = T | (() => T);
+
+export function readAccessor<T>(value: MaybeAccessor<T> | undefined | null, fallback: T): T {
+    if (value == null) return fallback;
+    return typeof value === "function" ? (value as () => T)() : value;
+}
+
+/** Reactive PageLayout/PageHeader prop — survives wrapProps (uses a dummy param). */
+export function accessor<T>(fn: () => T): (_: unknown) => T {
+    return (_: unknown) => fn();
+}