docs: Add a light border to h2s (#24554)

I was finding hard to navigate the "Configuring Zed" page with just
white space creating a boundary between the different chunks of content.
I think a slight border below the h2 heading helps a lot with that!

Release Notes:

- N/A

docs/theme/css/variables.css

@@ -98,7 +98,7 @@
   --title-color: hsl(220, 92%, 80%);
 
   --border: hsl(220, 13%, 20%);
-  --border-light: hsl(220, 13%, 90%);
+  --border-light: hsl(220, 13%, 15%);
   --border-hover: hsl(220, 13%, 40%);
 
   --media-bg: hsl(220, 13%, 8%);