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.py

@@ -0,0 +1,13 @@
+from textual.app import App
+from textual.widgets import Static
+
+
+class ContentAlignApp(App):
+    def compose(self):
+        yield Static("With [i]content-align[/] you can...", id="box1")
+        yield Static("...[b]Easily align content[/]...", id="box2")
+        yield Static("...Horizontally [i]and[/] vertically!", id="box3")
+
+
+app = ContentAlignApp(css_path="content_align.css")
+app.run()