Add keyboard focus-visible indicator for nav links

Agent-Logs-Url: https://github.com/PALP31/Triticum-tale-web/sessions/18319cc2-52c8-4e19-b674-4e530998975a

Co-authored-by: PALP31 <180587161+PALP31@users.noreply.github.com>

Author: Copilot

docs/custom.scss

@@ -16,6 +16,11 @@ h2 {
   text-decoration: underline;
 }
 
+.navbar .navbar-nav .nav-link:focus-visible {
+  outline: 2px solid $navbar-hl;
+  outline-offset: 2px;
+}
+
 .hero {
   padding: 3rem 1.5rem;
   margin: 1.5rem 0 2rem;