build(docs-infra): implement `@reviewed` tag definition (#40582)

The `@reviewed` tag can be used to specify the date when the
document was last reviewed.

PR Close #40582

aio/src/styles/2-modules/_guides.scss

@@ -0,0 +1,6 @@
+.reviewed {
+  color: lighten($darkgray, 10);
+  @include font-size(13);
+  font-style: italic;
+  text-align: right;
+}

aio/src/styles/2-modules/_modules-dir.scss

@@ -18,6 +18,7 @@
    @import 'errors';
    @import 'features';
    @import 'filetree';
+   @import 'guides';
    @import 'heading-anchors';
    @import 'hr';
    @import 'images';

aio/tools/transforms/content-package/tag-defs/reviewed.js

@@ -0,0 +1,21 @@
+/**
+ * @reviewed <date-string>
+ *
+ * Use this tag-def to indicate the date when this document was last reviewed.
+ * The date-string will be passed to the `Date()` constructor.
+ */
+module.exports = function(createDocMessage) {
+  return {
+    name: 'reviewed',
+    transforms(doc, tag, value) {
+      if (!/^\d{4}-\d{1,2}-\d{1,2}/.test(value.trim())) {
+        throw new Error(createDocMessage(
+          `Tag Error: @${tag.tagName} tag is missing the required date in the form "yyyy-mm-dd" but got "${value}"\n`,
+          doc));
+      }
+      return {
+        date: new Date(value),
+      };
+    }
+  };
+};

aio/tools/transforms/templates/content.template.html

@@ -3,12 +3,13 @@
 {% set relativePath = doc.fileInfo.relativePath %}
 {% if doc.title %}{$ ('# ' + doc.title.trim()) | marked $}{% endif %}
 {% if '/' in relativePath or 'docs.md' in relativePath %}
-  <div class="github-links">
-    {$ github.githubEditLink(doc, versionInfo) $}
-  </div>
+<div class="github-links">
+  {$ github.githubEditLink(doc, versionInfo) $}
+</div>
 {% endif %}
 {% block content %}
 <div class="content">
-{$ doc.description | marked $}
+  {$ doc.description | marked $}
+  {% if doc.reviewed %}<div class="reviewed">Last reviewed on {$ doc.reviewed.date.toDateString() $}</div>{% endif %}
 </div>
 {% endblock %}