Add example for all scrollbar size rules.

docs/examples/styles/scrollbar_size2.css

@@ -0,0 +1,18 @@
+Container {
+    width: 1fr;
+}
+
+#v1 {
+    scrollbar-size: 5 1;
+    background: red 20%;
+}
+
+#v2 {
+    scrollbar-size-vertical: 1;
+    background: green 20%;
+}
+
+#v3 {
+    scrollbar-size-horizontal: 5;
+    background: blue 20%;
+}

docs/examples/styles/scrollbar_size2.py

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+from textual.app import App
+from textual.containers import Horizontal, Container
+from textual.widgets import Label
+
+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 ScrollbarApp(App):
+    def compose(self):
+        yield Horizontal(
+            Container(Label(TEXT * 5), id="v1"),
+            Container(Label(TEXT * 5), id="v2"),
+            Container(Label(TEXT * 5), id="v3"),
+        )
+
+
+app = ScrollbarApp(css_path="scrollbar_size2.css")