Fix tree rendering

Fixes some tree rendering problems where lines were not properly drawn
at some levels during some circumstances.

* Change the API back to only allow one root.
* Now uses a stack based approach to rendering instead of recursion.
* Removes the need for measuring the whole tree in advance.
  Leave this up to each child to render.

docs/input/widgets/barchart.md

@@ -1,5 +1,5 @@
 Title: Bar Chart
-Order: 1
+Order: 20
 ---
 
 Use `BarChart` to render bar charts to the console.

docs/input/widgets/calendar.md

@@ -1,5 +1,5 @@
 Title: Calendar
-Order: 3
+Order: 40
 RedirectFrom: calendar
 ---
 

docs/input/widgets/canvas-image.md

@@ -1,5 +1,5 @@
 Title: Canvas Image
-Order: 6
+Order: 70
 ---
 
 To add [ImageSharp](https://github.com/SixLabors/ImageSharp) superpowers to 

docs/input/widgets/canvas.md

@@ -1,5 +1,5 @@
 Title: Canvas
-Order: 5
+Order: 60
 ---
 
 `Canvas` is a widget that allows you to render arbitrary "pixels" 

docs/input/widgets/figlet.md

@@ -1,5 +1,5 @@
 Title: Figlet
-Order: 4
+Order: 50
 RedirectFrom: figlet
 ---
 

docs/input/widgets/rule.md

@@ -1,5 +1,5 @@
 Title: Rule
-Order: 2
+Order: 30
 RedirectFrom: rule
 ---
 

docs/input/widgets/tree.md

@@ -0,0 +1,70 @@
+Title: Tree
+Order: 10
+---
+
+The `Tree` widget can be used to render hierarchical data.
+
+<img src="../assets/images/tree.png" style="width: 100%;" />
+
+# Usage
+
+```csharp
+// Create the tree
+var tree = new Tree("Root");
+
+// Add some nodes
+var foo = tree.AddNode("[yellow]Foo[/]");
+var table = foo.AddNode(new Table()
+    .RoundedBorder()
+    .AddColumn("First")
+    .AddColumn("Second")
+    .AddRow("1", "2")
+    .AddRow("3", "4")
+    .AddRow("5", "6"));
+
+table.AddNode("[blue]Baz[/]");
+foo.AddNode("Qux");
+
+var bar = tree.AddNode("[yellow]Bar[/]");
+bar.AddNode(new Calendar(2020, 12)
+    .AddCalendarEvent(2020, 12, 12)
+    .HideHeader());
+
+// Render the tree
+AnsiConsole.Render(root);
+```
+
+# Collapsing nodes
+
+```csharp
+root.AddNode("Label").Collapsed();
+```
+
+# Appearance
+
+## Style
+
+```csharp
+var root = new Tree("Root")
+    .Style("white on red");
+```
+
+## Guide lines
+
+```csharp
+// ASCII guide lines
+var root = new Tree("Root")
+    .Guide(TreeGuide.Ascii);
+
+// Default guide lines
+var root = new Tree("Root")
+    .Guide(TreeGuide.Line);
+
+// Double guide lines
+var root = new Tree("Root")
+    .Guide(TreeGuide.DoubleLine);
+
+// Bold guide lines
+var root = new Tree("Root")
+    .Guide(TreeGuide.BoldLine);
+```