Add layout documentation

docs/input/widgets/layout.md

@@ -0,0 +1,64 @@
+Title: Layout
+Order: 45
+Description: "Use **Layout** to layout widgets in the terminal."
+Reference: T:Spectre.Console.Layout
+
+---
+
+Use `Layout` to layout widgets in the terminal.
+
+<?# AsciiCast cast="layout" /?>
+
+## Usage
+
+```csharp
+// Create the layout
+var layout = new Layout("Root")
+    .SplitColumns(
+        new Layout("Left"),
+        new Layout("Right")
+            .SplitRows(
+                new Layout("Top"),
+                new Layout("Bottom")));
+
+// Update the left column
+layout["Left"].Update(
+    new Panel(
+        Align.Center(
+            new Markup("Hello [blue]World![/]"),
+            VerticalAlignment.Middle))
+        .Expand());
+
+// Render the layout
+AnsiConsole.Write(layout);
+```
+
+## Setting minimum size
+
+```csharp
+layout["Left"].MinimumSize(10);
+```
+
+## Setting ratio
+
+```csharp
+layout["Left"].Ratio(2);
+```
+
+## Settings explicit size
+
+```csharp
+layout["Left"].Size(32);
+```
+
+## Hide layout
+
+```csharp
+layout["Left"].Invisible();
+```
+
+## Show layout
+
+```csharp
+layout["Left"].Visible();
+```