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/vertical_layout_scrolled.css

@@ -0,0 +1,8 @@
+Screen {
+    layout: vertical;
+}
+
+.box {
+    height: 14;
+    border: solid green;
+}