docs: add scroll to top button

Add ScrollToTop component that appears when the user scrolls down
the page, allowing quick navigation back to the top of the document.

Author: brnrdog

docs-website/src/App.res

@@ -102,6 +102,7 @@ let make = () => {
               {routes}
               <PageNavigation />
             </div>
+            <ScrollToTop />
           </AppLayout>,
         ]
       }

docs-website/src/components/ScrollToTop.res

@@ -0,0 +1,47 @@
+open Xote
+open Basefn
+
+// Get scroll position
+let getScrollY: unit => float = %raw(`function() { return window.scrollY || 0 }`)
+
+// Scroll to top
+let scrollToTop: unit => unit = %raw(`function() { window.scrollTo({ top: 0, behavior: 'smooth' }) }`)
+
+// Add scroll listener
+let addScrollListener: (unit => unit) => unit => unit = %raw(`function(callback) {
+  window.addEventListener('scroll', callback)
+  return function() { window.removeEventListener('scroll', callback) }
+}`)
+
+@jsx.component
+let make = () => {
+  let isVisible = Signal.make(false)
+
+  let _ = Effect.run(() => {
+    let cleanup = addScrollListener(() => {
+      let scrollY = getScrollY()
+      Signal.set(isVisible, scrollY > 300.0)
+    })
+    Some(cleanup)
+  })
+
+  let handleClick = _ => {
+    scrollToTop()
+  }
+
+  Component.signalFragment(
+    Computed.make(() => {
+      if Signal.get(isVisible) {
+        [
+          <div style="position: fixed; bottom: 2rem; right: 2rem; z-index: 1000;">
+            <Button variant={Secondary} onClick={handleClick}>
+              <Icon name={ChevronUp} size={Md} />
+            </Button>
+          </div>,
+        ]
+      } else {
+        []
+      }
+    }),
+  )
+}