refactor: rename DTree to DValue

site/doc/pages/component.txt

@@ -1,5 +1,5 @@
 :sig
-String component(String name, [DTree props], [Request& context])
+String component(String name, [DValue props], [Request& context])
 
 :see
 >ob
@@ -37,7 +37,7 @@ When a component unit defines `ONCE(Request& context)`, the runtime calls that h
 Default component handler:
 
 ```cpp
-DTree props;
+DValue props;
 props["title"] = "Status";
 
 <><?: component("workspace/panel", props, context) ?></>
@@ -46,7 +46,7 @@ props["title"] = "Status";
 Named component handler:
 
 ```cpp
-DTree props;
+DValue props;
 props["title"] = "System";
 props["body"] = "Healthy";
 
@@ -76,7 +76,7 @@ COMPONENT:BODY(Request& context)
 Preparing props in C++ before rendering:
 
 ```cpp
-DTree props;
+DValue props;
 props["items"][0] = "alpha";
 props["items"][1] = "beta";
 props["items"][2] = "gamma";