Create & Register workflows at runtime

We need a section about the ability to define a workflow from code and then register it with the runtime. For example:

```csharp
// Define workflow.
var workflow = new Workflow(new Flowchart())
{
Metadata = { DefinitionId = "MyDynamicWorkflow" }
Name = "MyDynamicWorkflow"
};

// Register workflow.
_runtime.register(workflow);

// Execute workflow.
_runtime.StartWorkflow("MyDynamicWorkflow", new StartWorkflowOptions());
```