build(aio): add checkContentRules processor (#22759)

This processor will enable us to write rules about
how the content should appear, such as:

* no headings in markdown content
* only one sentence per line
* no single character parameter names
* etc.

PR Close #22759

aio/tools/transforms/angular-base-package/index.js

@@ -30,6 +30,7 @@ module.exports = new Package('angular-base', [
   .processor(require('./processors/fixInternalDocumentLinks'))
   .processor(require('./processors/copyContentAssets'))
   .processor(require('./processors/renderLinkInfo'))
+  .processor(require('./processors/checkContentRules'))
 
   // overrides base packageInfo and returns the one for the 'angular/angular' repo.
   .factory('packageInfo', function() { return require(path.resolve(PROJECT_ROOT, 'package.json')); })