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/layers.css

@@ -0,0 +1,22 @@
+Screen {
+    layout: center;
+    layers: below above;
+}
+
+Static {
+    width: 28;
+    height: 8;
+    color: auto;
+    content-align: center middle;
+}
+
+#box1 {
+    background: darkcyan;
+    layer: above;
+}
+
+#box2 {
+    layer: below;
+    background: orange;
+    offset: 12 6;
+}