MAINTAIN: Update docs to reflect output dir of page (#912)

docs/start/build.md

@@ -52,16 +52,15 @@ You can generate a standalone HTML file for a single page of the Jupyter Book us
 jupyter-book build path/to/mypage.ipynb
 ```
 
-This will execute your content and output the proper HTML in a
-`_build/html` folder.
+This will execute your content and output the proper HTML in a `_build/_page/html/<mypage>` folder.
+If the file is in a subdirectory relative to the `_build` folder, the HTML  will be in a `_build/_page/html/<subdirectory-mypage>` folder.
 
 Your page will be called `mypage.html`. This will work
 for any {doc}`content source file <../content-types/index>` that is supported by Jupyter Book.
 
 ```{note}
-Users should note that building **single pages** in the context of a larger project,
-can trigger warnings and incomplete links. For example, building `docs/start/overview.md` will
-issue a bunch of `unknown document`,`term not in glossary`, and `undefined links` warnings.
+Users should note that building **single pages** in the context of a larger project, can trigger warnings and incomplete links.
+For example, building `docs/start/overview.md` will issue a bunch of `unknown document`,`term not in glossary`, and `undefined links` warnings.
 ```
 
 ## Page caching