Update Docs Readme

Added some more context to the docs readme for
folks looking to contribute. Also added the license
so its clear what the license of the docs site is. As
a small tweak I removed a duplicate string value and
use the Constant.

docs/src/Pipelines/ColorsPipeline.cs

@@ -9,15 +9,13 @@ namespace Docs.Pipelines
 {
     public class ColorsPipeline : Pipeline
     {
-        public const string Url = "https://raw.githubusercontent.com/spectresystems/spectre.console/main/resources/scripts/Generator/Data/colors.json";
-
         public ColorsPipeline()
         {
             InputModules = new ModuleList
             {
                 new ExecuteConfig(
                     Config.FromContext(ctx => {
-                        return new ReadWeb(Url);
+                        return new ReadWeb(Constants.Colors.Url);
                     }))
             };
 
@@ -32,4 +30,4 @@ namespace Docs.Pipelines
             };
         }
     }
-}
+}