docs: update README and author guide about doc build errors (#19276)

- tells reader about `yarn serve-and-sync`.
- directs reader to look to docs-style-guide if get doc build error.
- update docs-style-guide to warn about ignored code files.

PR Close #19276

aio/README.md

@@ -52,7 +52,7 @@ For more details see #16745.
 
 ## Guide to authoring
 
-There are two types of content in the documentatation:
+There are two types of content in the documentation:
 
 * **API docs**: descriptions of the modules, classes, interfaces, decorators, etc that make up the Angular platform.
 API docs are generated directly from the source code.
@@ -107,8 +107,16 @@ yarn start
 yarn docs-watch
 ```
 
+>Alternatively, try the consolidated `serve-and-sync` command that builds, watches and serves in the same terminal window
+```bash
+yarn serve-and-sync
+```
+
 * Open a browser at https://localhost:4200/ and navigate to the document on which you want to work.
 You can automatically open the browser by using `yarn start -- -o` in the first terminal.
 
 * Make changes to the page's associated doc or example files. Every time a file is saved, the doc will
 be regenerated, the app will rebuild and the page will reload.
+
+* If you get a build error complaining about examples or any other odd behavior, be sure to consult
+the [Authors Style Guide](https://angular.io/guide/docs-style-guide).

aio/content/guide/docs-style-guide.md

@@ -348,7 +348,7 @@ Here's the brief markup that produced that lengthy snippet:
 ```
 
 You identified the snippet's source file by setting the `path` attribute to sample folder's location _within_ `content/examples`.
-In this example, that path is  `docs-style-guide/src/app/app.module.ts`.  
+In this example, that path is  `docs-style-guide/src/app/app.module.ts`.
 
 You added a header to tell the reader where to find the file by setting the `title` attribute.
 Following convention, you set the `title` attribute to the file's location within the sample's root folder.
@@ -360,6 +360,23 @@ located in the `content/examples/docs-style-guide` directory.
 
 </div>
 
+<div class="alert is-important">
+
+The doc tooling reports an error if the file identified in the path does not exist **or is _git_-ignored**.
+
+Most `.js` files are _git_-ignored.
+If you want to include an ignored code file in your project and display it in a guide you must _un-ignore_ it.
+
+The preferred way to un-ignore a file is to update the `content/examples/.gitignore` like this:
+
+<code-example title="content/examples/.gitignore">
+  # my-guide
+  !my-guide/src/something.js
+  !my-guide/more-javascript*.js
+</code-example>
+
+</div>
+
 #### Code-example attributes 
 
 You control the _code-example_ output by setting one or more of its attributes: