Implement back to top fade in and out transitions (#4093)

* fade in and out

* Update back-to-top.tsx

* Update toc.tsx

* Create new-years-sit.md

---------

Co-authored-by: Dimitri POSTOLOV <dmytropostolov@gmail.com>

Author: hichemfantar

.changeset/new-years-sit.md

@@ -0,0 +1,5 @@
+---
+"nextra-theme-docs": patch
+---
+
+add the missing `transition-opacity` style for the `<BackToTop>` component

packages/nextra-theme-docs/src/components/back-to-top.tsx

@@ -30,7 +30,7 @@ export const BackToTop: FC<{
       disabled={hidden}
       className={({ disabled }) =>
         cn(
-          'x:flex x:items-center x:gap-1.5 x:cursor-pointer',
+          'x:flex x:items-center x:gap-1.5 x:cursor-pointer x:transition-opacity',
           'x:whitespace-nowrap', // Safari
           disabled ? 'x:opacity-0' : 'x:opacity-100',
           className