OGPメタタグを改善

- og:image:typeとog:image:altプロパティを追加
- twitter:image:altプロパティを追加
- メタデータの完全性を向上

Author: Taka-bow

deck/overrides/main.html

@@ -12,8 +12,10 @@
   <meta property="og:title" content="{% if page.title %}{{ page.title }} - {% endif %}{{ config.site_name }}">
   <meta property="og:description" content="{% if page.description %}{{ page.description }}{% else %}{{ config.site_description }}{% endif %}">
   <meta property="og:image" content="{{ config.site_url }}assets/og-image.png">
+  <meta property="og:image:type" content="image/png">
   <meta property="og:image:width" content="1200">
   <meta property="og:image:height" content="630">
+  <meta property="og:image:alt" content="{{ config.site_name }}">
   <meta property="og:site_name" content="{{ config.site_name }}">
   <meta property="og:locale" content="ja_JP">
 
@@ -27,6 +29,7 @@
   <meta name="twitter:title" content="{% if page.title %}{{ page.title }} - {% endif %}{{ config.site_name }}">
   <meta name="twitter:description" content="{% if page.description %}{{ page.description }}{% else %}{{ config.site_description }}{% endif %}">
   <meta name="twitter:image" content="{{ config.site_url }}assets/og-image.png">
+  <meta name="twitter:image:alt" content="{{ config.site_name }} - {{ config.site_description }}">
 
   <!-- Additional Meta Tags -->
   <link rel="canonical" href="{{ config.site_url }}{% if page.url %}{{ page.url }}{% endif %}">