Add SEO improvements: canonical, hreflang, schema fixes, robots.txt

- Fix breadcrumb schema position bug (3 → 2) for Google Rich Results
- Override article/post schemas to use post image instead of site logo
- Fix og:type to emit "article" on post pages with published/modified times
- Add canonical link and hreflang alternate tags (incl. x-default) for bilingual site
- Add organizationName/organizationLogo params for valid Organization schema
- Add robots.txt with sitemap pointer
- Add preconnect hint for analytics host

layouts/partials/seo/structured/breadcrumb.html

@@ -0,0 +1,21 @@
+<script type="application/ld+json">
+{
+  "@context": "http://schema.org",
+  "@type": "BreadcrumbList",
+  "itemListElement": [{
+        "@type": "ListItem",
+        "position": 1,
+        "item": {
+          "@id": "{{ .Site.BaseURL }}",
+          "name": "home"
+        }
+    },{
+        "@type": "ListItem",
+        "position": 2,
+        "item": {
+          "@id": "{{ .Permalink }}",
+          "name": "{{ .Title | humanize }}"
+        }
+    }]
+}
+</script>