Initial layout guide stuff (#748)

* Initial layout guide stuff

* More docs on layout - grid

* Continuing grid docs

* Grid gutter and spans guide

* Improvements to layout docs for horizontal, vertical, dock, and begin describing layers

* Update center layout example to reflect new yield order

* More updates to layout guide, mostly offset stuff

* More layout guide, "Putting it all together"

* Updates to layout guide page

* Small rewording of dock layout in guide

* Apostrophe

* Typo

* Small design tweak to combining layouts example

* Typos, tidying up

* Small reword

* Some updates to docs/guide/layout/grid

* calc fix

Co-authored-by: Will McGugan <willmcgugan@gmail.com>

docs/examples/guide/layout/grid_layout7_gutter.py

@@ -0,0 +1,17 @@
+from textual.app import App, ComposeResult
+from textual.widgets import Static
+
+
+class GridLayoutExample(App):
+    def compose(self) -> ComposeResult:
+        yield Static("One", classes="box")
+        yield Static("Two", classes="box")
+        yield Static("Three", classes="box")
+        yield Static("Four", classes="box")
+        yield Static("Five", classes="box")
+        yield Static("Six", classes="box")
+
+
+app = GridLayoutExample(css_path="grid_layout7_gutter.css")
+if __name__ == "__main__":
+    app.run()