docs: fix banner accessibility and hover color contrast (qodo-ai#2245)

Apply WCAG AA compliant colors for the announcement banner:
- Light mode: #1d4ed8 (6.70:1 contrast with white text)
- Dark mode: #2563eb (5.17:1 contrast with white text, 3.31:1 vs header)
- Fix link hover color inheriting accent color instead of staying white

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: naorpeled

docs/docs/css/custom.css

@@ -105,3 +105,18 @@
     border-color: rgba(226, 232, 240, 0.2);
     outline-color: rgba(226, 232, 240, 0.3);
 }
+
+.md-banner {
+    background-color: #1d4ed8;
+}
+
+[data-md-color-scheme="slate"] .md-banner {
+    background-color: #2563eb;
+}
+
+.md-banner .md-typeset a,
+.md-banner .md-typeset a:hover,
+.md-banner .md-typeset a:focus {
+    color: currentColor;
+    text-decoration: underline;
+}