Management API model mapping take two (#15667)

* Re-modelling API models (take two)

* Do not use "content" - use "document" and "media" instead.

* Move "item" endpoints to their own silo

* Use "Id", not "Key"

* Regenerate OpenApi.json

* Rename user start node IDs from "content" to "document"

* Regererate OpenApi.json after forward merge

* update importmap

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Co-authored-by: Mads Rasmussen <madsr@hey.com>

src/Umbraco.Core/Constants-Web.cs

@@ -68,6 +68,7 @@ public static partial class Constants
         {
             public const string Tree = "tree";
             public const string RecycleBin = "recycle-bin";
+            public const string Item = "item";
         }
 
         public static class AttributeRouting