add buildDate for docs (#5447)

docs/blog/themes/hugo-theme-learn/layouts/partials/meta.html

 <meta name="description" content="{{ with .Description }}{{ . }}{{ else }}{{ with .Site.Params.description }}{{ . }}{{ end }}{{ end }}">
+    <meta name="buildDate" content="{{ .Hugo.BuildDate }}">
 {{ with .Site.Params.author }}<meta name="author" content="{{ . }}">{{ end }}

docs/community/themes/hugo-theme-learn/layouts/partials/meta.html

 <meta name="description" content="{{ with .Description }}{{ . }}{{ else }}{{ with .Site.Params.description }}{{ . }}{{ end }}{{ end }}">
+    <meta name="buildDate" content="{{ .Hugo.BuildDate }}">
 {{ with .Site.Params.author }}<meta name="author" content="{{ . }}">{{ end }}

docs/document/themes/hugo-theme-learn/layouts/partials/meta.html

 <meta name="description" content="{{ with .Description }}{{ . }}{{ else }}{{ with .Site.Params.description }}{{ . }}{{ end }}{{ end }}">
+    <meta name="buildDate" content="{{ .Hugo.BuildDate }}">
 {{ with .Site.Params.author }}<meta name="author" content="{{ . }}">{{ end }}