fix(schema): use page URL for mainEntityOfPage instead of "true"

The Article JSON-LD set "mainEntityOfPage": "true" on every non-home page.
schema.org/mainEntityOfPage expects a CreativeWork or a URL — a boolean
string is invalid. Emit the page's canonical URL, mirroring the adjacent
"url" : {{ .Permalink }}.

Fixes #2971

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

layouts/partials/schema.html

@@ -41,7 +41,7 @@
     {{ else }}
     {{ with .Params.tags }}"keywords": {{ . }},{{ end }}
     {{ end }}
-    "mainEntityOfPage": "true",
+    "mainEntityOfPage": {{ .Permalink }},
     "wordCount": "{{ .WordCount }}"
   }{{ if site.Params.enableStructuredBreadcrumbs | default false }},
   {