Updated sidebar documentation to clarify nesting (#626)

The documentation for how to create nested sidebars was unclear, and I could not find it anywhere else. After experimenting, I discovered the tactic and felt it should be in the docs.

Author: ciel

docs/more-pages.md

@@ -51,6 +51,10 @@ Create the `_sidebar.md`:
 
 You need to create a `.nojekyll` in `./docs` to prevent GitHub Pages from ignoring files that begin with an underscore.
 
+## Nested Sidebars
+
+You may want the sidebar to update with only navigation to reflect the current directory. This can be done by adding a `_sidebar.md` file to each folder.
+
 `_sidebar.md` is loaded from each level directory. If the current directory doesn't have `_sidebar.md`, it will fall back to the parent directory. If, for example, the current path is `/guide/quick-start`, the `_sidebar.md` will be loaded from `/guide/_sidebar.md`.
 
 You can specify `alias` to avoid unnecessary fallback.
@@ -66,6 +70,8 @@ You can specify `alias` to avoid unnecessary fallback.
 </script>
 ```
 
+!> You can create a `README.md` file in a subdirectory to use it as the landing page for the route.
+
 ## Table of Contents
 
 Once you've created `_sidebar.md`, the sidebar content is automatically generated based on the headers in the markdown files.