Docs scrollbar gutter (#695)

* Add scrollbar-gutter docs

* Add scrollbar-gutter docs

* Add syntax and values section

* Rewording

docs/examples/styles/scrollbar_gutter.py

@@ -0,0 +1,28 @@
+from textual.app import App
+from textual.widgets import Static
+
+TEXT = """I must not fear.
+Fear is the mind-killer.
+Fear is the little-death that brings total obliteration.
+I will face my fear.
+I will permit it to pass over me and through me.
+And when it has gone past, I will turn the inner eye to see its path.
+Where the fear has gone there will be nothing. Only I will remain."""
+
+
+class ScrollbarGutterApp(App):
+    CSS = """
+    Screen {
+        scrollbar-gutter: stable;
+    }
+    Static {
+        color: floralwhite;
+        background: darkmagenta;
+    }
+    """
+
+    def compose(self):
+        yield Static(TEXT)
+
+
+app = ScrollbarGutterApp()