Fix sidebar navigation text contrast in light mode (#902)

* Fixing the action link

* Fix sidebar navigation text contrast in light mode

- Use --sl-color-text-invert instead of --sl-color-white for active
  sidebar item, fixing near-black text on dark purple background
  (contrast ~2.5:1 → ~7.1:1)
- Bump inactive sidebar links to font-weight 500 for better readability

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

---------

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: aaronpowell

website/src/content/docs/learning-hub/index.md

@@ -6,7 +6,7 @@ hero:
   tagline: 'Curated articles, walkthroughs, and reference material to help you unlock everything you can do with GitHub Copilot'
   actions:
     - text: Start with Fundamentals
-      link: /learning-hub/what-are-agents-skills-instructions/
+      link: what-are-agents-skills-instructions/
       icon: right-arrow
 sidebar:
   hidden: true

website/src/styles/starlight-overrides.css

@@ -41,10 +41,14 @@
   --sl-color-black: #f0f0f5;
 }
 
-/* ── Sidebar active item ───────────────────────────────────── */
+/* ── Sidebar readability ───────────────────────────────────── */
+nav[aria-label="Main"] a {
+  font-weight: 500 !important;
+}
+
 nav[aria-label="Main"] a[aria-current="page"] {
-  color: var(--sl-color-white) !important;
-  font-weight: 600;
+  color: var(--sl-color-text-invert) !important;
+  font-weight: 600 !important;
   border-inline-start-color: var(--sl-color-accent) !important;
 }