Document keys of global variable `{{ jekyll }}` (#9653)

Merge pull request 9653

docs/_data/jekyll_variables.yml

@@ -16,6 +16,9 @@ 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: jekyll
+    description: >-
+      Jekyll-centric information will be available here. See below for details.
   - name: theme
     description: >-
       Theme-gem specific information as defined in the theme's gemspec. Useful for rendering
@@ -154,6 +157,12 @@ 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.
 
+jekyll:
+  - name: jekyll.version
+    description: Version of Jekyll used to build the site.
+  - name: jekyll.environment
+    description: Value assigned to environment variable <code>JEKYLL_ENV</code> during a build.
+
 theme:
   - name: theme.root
     description: Absolute path to the theme-gem.

docs/_docs/variables.md

@@ -34,6 +34,10 @@ The following is a reference of the available data.
   </p>
 </div>
 
+## Jekyll Variables
+
+{% include docs_variables_table.html scope=site.data.jekyll_variables.jekyll %}
+
 ## Theme Variables {%- include docs_version_badge.html version="4.3.0" -%}
 {: #theme-variables }