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/head_custom.html

@@ -1,3 +1,11 @@
+    <link rel="canonical" href="{{ .Permalink }}" />
+    {{ if .IsTranslated }}
+    {{ range .AllTranslations }}
+    <link rel="alternate" hreflang="{{ .Lang }}" href="{{ .Permalink }}" />
+    {{ end }}
+    {{ range .AllTranslations }}{{ if eq .Lang "en" }}<link rel="alternate" hreflang="x-default" href="{{ .Permalink }}" />{{ end }}{{ end }}
+    {{ end }}
+    <link rel="preconnect" href="https://pythagore.p0m.fr" />
     <script async defer data-website-id="714f2f10-3a65-4c5b-971f-7cea9eac1ecd" src="https://pythagore.p0m.fr/pythagore.js"></script>
     <link rel="stylesheet" href="{{ "css/custom.css" | absURL }}" />
     <style>