build(aio): move doc-gen stuff from angular.io (#14097)

tools/docs/angular.io-package/rendering/trimBlankLines.js

@@ -0,0 +1,15 @@
+module.exports = function() {
+  return {
+    name: 'trimBlankLines',
+    process: function(str) {
+      var lines = str.split(/\r?\n/);
+      while (lines.length && (lines[0].trim() === '')) {
+        lines.shift();
+      }
+      while (lines.length && (lines[lines.length - 1].trim() === '')) {
+        lines.pop();
+      }
+      return lines.join('\n');
+    }
+  };
+};