feat: add canonical URL and og:url

Docus doesn't generate canonical links or og:url natively. This
branch has no @nuxtjs/sitemap/nuxt-site-config, so the plugin reads
the base URL from docus.url in app.config.ts (already includes the
/fr prefix) instead of hardcoding it, and combines it with the
current route path.

plugins/canonical.ts

@@ -0,0 +1,14 @@
+export default defineNuxtPlugin(() => {
+  const route = useRoute()
+  const { docus } = useAppConfig()
+  const canonicalUrl = computed(() => `${docus.url}${route.path}`)
+
+  useHead({
+    link: [
+      { rel: 'canonical', href: canonicalUrl }
+    ],
+    meta: [
+      { property: 'og:url', content: canonicalUrl }
+    ]
+  })
+})