code: protected Map<String, Object> groupIndexMap(
              final List<DocWorkUnit> workUnitList,
              final List<Map<String, String>> groupMaps
      ) {
          //
          // root -> data -> { summary -> y, filename -> z }, etc
          //      -> groups -> group1, group2, etc.
          Map<String, Object> root = new HashMap<>();
  
          Collections.sort(workUnitList);
  
          List<Map<String, String>> data = new ArrayList<>();
          workUnitList.stream().forEach(workUnit -> data.add(indexDataMap(workUnit)));
  
          root.put("data", data);
          root.put("groups", groupMaps);
          root.put("timestamp", getBuildTimeStamp());
          root.put("version", getBuildVersion());
  
          return root;
      }
docs: Helpful function to create the php index.  Given all of the already run DocWorkUnits,
  create the high-level grouping data listing individual features by group.
  
  @param workUnitList
  @return The map used to populate the index template used by this doclet.