enable snippet compiler on reference docs for new-types

The change in unin-types-spec.md is workaround. It should be `sc:fail` instead of
`sc:nocompile`, but it causes generateReferenceDocumentation task to
fail with MissingCoreLibraryException.

Author: i10416

docs/_docs/reference/new-types/union-types-spec.md

@@ -156,7 +156,7 @@ The members of a union type are the members of its join.
 The following code does not typecheck, because method `hello` is not a member of
 `AnyRef` which is the join of `A | B`.
 
-```scala
+```scala sc:nocompile
 trait A { def hello: String }
 trait B { def hello: String }
 

project/Build.scala

@@ -2360,6 +2360,7 @@ object ScaladocConfigs {
       .add(DocumentSyntheticTypes(true))
       .add(SnippetCompiler(List(
         s"${dottyLibRoot}/scala=compile",
+        "docs/_docs/reference/new-types=compile"
       )))
       .add(SiteRoot("docs"))
       .add(ApiSubdirectory(true))