Change more examples from Belt to RescriptCore

Author: fhammerschmidt

pages/docs/manual/latest/build-external-stdlib.mdx

@@ -42,7 +42,7 @@ Now the compiled JS code will import using the path defined by `external-stdlib`
 <CodeTab labels={["ReScript", "JS output"]}>
 
 ```res
-Belt.Array.forEach([1, 2, 3], num => Js.log(num))
+Array.forEach([1, 2, 3], num => Js.log(num))
 ```
 
 ```js

pages/docs/manual/latest/generate-converters-accessors.mdx

@@ -88,8 +88,8 @@ type pet = {name: string}
 let pets = [{name: "bob"}, {name: "bob2"}]
 
 pets
- ->Belt.Array.map(name)
- ->Js.Array2.joinWith("&")
+ ->Array.map(name)
+ ->Array.joinWith("&")
  ->Js.log
 ```