Simplify social.html. (#24483)

This also fixes the wrong usage of `site.description` instead of `page.description` in meta description.

_includes/header.html

@@ -1,5 +1,7 @@
 <meta charset="utf-8">
 <meta name="viewport" content="width=device-width, initial-scale=1, shrink-to-fit=no">
+<meta name="description" content="{{ page.description | default: site.description }}">
+<meta name="author" content="{{ site.authors }}">
 <meta name="generator" content="Jekyll v{{ jekyll.version }}">
 
 <title>