fix: code container hiding navigation menu (#4707)

* fix: code container hiding navigation menu

code container and navigation menu z-index values was
same. Navigation menu was hidden behind code container
for smaller screen. Code container has pushed down with
the fix.

* fix: fixed review comments

Instead of hardcoded values, calculated z-index from the variable

packages/website/src/components/Playground.module.css

@@ -39,5 +39,5 @@
   width: 100%;
   height: calc(100% - var(--ifm-navbar-height));
   top: var(--ifm-navbar-height);
-  z-index: var(--ifm-z-index-fixed);
+  z-index: calc(var(--ifm-z-index-fixed) - 1);
 }