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/apps/user-guide/providers/main-nav.tsx

@@ -15,7 +15,7 @@ type MainNavProviderProps = {
 }
 
 export const MainNavProvider = ({ children }: MainNavProviderProps) => {
-  const isBrowser = useIsBrowser()
+  const { isBrowser } = useIsBrowser()
   const navigationDropdownItems = useMemo(
     () =>
       getNavDropdownItems({