Document keys of the `theme` drop

Follow-up to #9129

docs/_data/jekyll_variables.yml

@@ -16,6 +16,10 @@ global:
     description: >-
       Layout specific information + the <a href="/docs/front-matter/">front matter</a>.
       Custom variables set via front matter in layouts will be available here.
+  - name: theme
+    description: >-
+      Theme-gem specific information as defined in the theme's gemspec. Useful for rendering
+      information in the theme demo's "About" page, for example. See below for details.
   - name: content
     description: >-
       In layout files, the rendered content of the Post or Page being wrapped.
@@ -150,6 +154,20 @@ page:
       The previous post relative to the position of the current post in <code>site.posts</code>.
       Returns <code>nil</code> for the first entry.
 
+theme:
+  - name: theme.root
+    description: Absolute path to the theme-gem.
+  - name: theme.authors
+    description: Comma separated string composed of the authors of the theme-gem.
+  - name: theme.description
+    description: Description or summary of the theme-gem as specified in the theme gemspec. 
+  - name: theme.version
+    description: The version string of current theme.
+  - name: theme.dependencies
+    description: List of runtime dependencies of the theme.
+  - name: theme.metadata
+    description: A mapping of key-value pairs as defined in the theme gemspec. 
+
 paginator:
   - name: paginator.page
     description: The number of the current page

docs/_docs/variables.md

@@ -34,6 +34,10 @@ The following is a reference of the available data.
   </p>
 </div>
 
+## Theme
+
+{% include docs_variables_table.html scope=site.data.jekyll_variables.theme %}
+
 ## Paginator
 
 {% include docs_variables_table.html scope=site.data.jekyll_variables.paginator %}

lib/jekyll/drops/theme_drop.rb

@@ -3,7 +3,7 @@
 module Jekyll
   module Drops
     class ThemeDrop < Drop
-      delegate_methods   :root, :version
+      delegate_methods   :root
       delegate_method_as :runtime_dependencies, :dependencies
 
       def authors