docs: Add footer with links to pages (#50296)

This PR includes a little footer with handy links in all docs pages:

<img width="500" height="532" alt="Screenshot 2026-02-27 at 10  03@2x"
src="https://github.com/user-attachments/assets/f1beb998-ae73-41d1-8fe1-734def58b0c5"
/>

Release Notes:

- N/A

docs/theme/css/variables.css

@@ -27,6 +27,7 @@
   --border: hsl(220, 13%, 80%);
   --border-light: hsl(220, 13%, 90%);
   --border-hover: hsl(220, 13%, 70%);
+  --border-footer: hsl(220, 13%, 91%);
 
   --media-bg: hsl(50, 25%, 92%);
 
@@ -124,6 +125,7 @@
   --border: hsl(220, 13%, 20%);
   --border-light: hsl(220, 13%, 15%);
   --border-hover: hsl(220, 13%, 40%);
+  --border-footer: hsl(220, 13%, 12%);
 
   --media-bg: hsl(220, 13%, 8%);