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/examples/styles/content_align.css

@@ -0,0 +1,20 @@
+#box1 {
+    content-align: left top;
+    background: red;
+}
+
+#box2 {
+    content-align: center middle;
+    background: green;
+}
+
+#box3 {
+    content-align: right bottom;
+    background: blue;
+}
+
+Static {
+    height: 1fr;
+    padding: 1;
+    color: white;
+}