Docs content align (#700)

* Docs for content-align, some bug fixes for it too

* Make the example output a little clearer

* Remove app.run() from content-align docs example

* Improve wording

* Move content-align and scrollbar-gutter docs exmaples to split css/py files

* Remove app.run() from scrollbar-gutter docs example

docs/styles/scrollbar_gutter.md

@@ -1,4 +1,4 @@
-# Scrollbar gutter
+# Scrollbar-gutter
 
 The `scrollbar-gutter` rule allows authors to reserve space for the vertical scrollbar.
 
@@ -28,6 +28,12 @@ terminal window.
     --8<-- "docs/examples/styles/scrollbar_gutter.py"
     ```
 
+=== "scrollbar_gutter.css"
+
+    ```scss
+    --8<-- "docs/examples/styles/scrollbar_gutter.css"
+    ```
+
 === "Output"
 
     ```{.textual path="docs/examples/styles/scrollbar_gutter.py"}