Update reference pages for prerelease v1.9.31

Author: github-actions[bot]

docs/cli/cli-info.json

@@ -1,5 +1,5 @@
 {
-  "version": "1.9.27",
+  "version": "1.9.31",
   "commands": [
     {
       "name": "render",

docs/reference/formats/epub.json

@@ -119,10 +119,6 @@
         "name": "contributor",
         "description": "Contributors to this publication."
       },
-      {
-        "name": "subject",
-        "description": "The subject of the publication."
-      },
       {
         "name": "type",
         "description": "Text describing the specialized type of this publication.\n\nAn informative registry of specialized EPUB Publication \ntypes for use with this element is maintained in the \n[TypesRegistry](https://www.w3.org/publishing/epub32/epub-packages.html#bib-typesregistry), \nbut Authors may use any text string as a value.\n"
@@ -414,6 +410,10 @@
     "name": "metadata",
     "title": "Metadata",
     "options": [
+      {
+        "name": "subject",
+        "description": "The document subject"
+      },
       {
         "name": "date-meta",
         "description": "Sets the date metadata for the document"