docs: DX and performance improvements in API reference (#9430)

- Improve scroll behavior between active sections
- Improve lag when clicking on a sidebar item
- Refactor internal working of the `SidebarProvider` to find active items faster.
- Use Next.js's `useRouter` hook for changing the hash (since they added the option to disable scroll)
- Change `isBrowser` from a hook to a provider since it's widely used across applications.
- Other general improvements and fixes.

Closes DOCS-952

www/packages/docs-ui/src/components/Toc/index.tsx

@@ -16,7 +16,7 @@ import { TocMenu } from "./Menu"
 export const Toc = () => {
   const [items, setItems] = useState<ToCItemUi[]>([])
   const [showMenu, setShowMenu] = useState(false)
-  const isBrowser = useIsBrowser()
+  const { isBrowser } = useIsBrowser()
   const { items: headingItems, activeItemId } = useActiveOnScroll({})
   const [maxHeight, setMaxHeight] = useState(0)
   const { scrollableElement } = useScrollController()