Refactor navigation to Tabs+Languages, add contact page and Inter font

Switch primary navigation to Tabs containing Languages for multi-topic
extensibility. Add contact page with project links, issue reporting,
and contribution guide. Set Inter as the sans-serif font for English
text and add custom CSS for description sizing and logo rendering.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

custom.css

@@ -0,0 +1,13 @@
+/* Scale up logo rendering */
+img[alt*="logo" i],
+img[src*="logo"],
+img[src*="favicon"] {
+  max-height: 72px !important;
+  height: auto !important;
+  width: auto !important;
+}
+
+/* Page description (frontmatter) - match body text size */
+header .text-lg {
+  font-size: 1rem;
+}