Rewrite dotty.tools.dottydoc.staticsite.Yaml in Java

To work properly, `TypeReference` relies on using runtime reflection to
get the generic arguments it was instantiated with, this does not work
currently with Dotty, because we're not emitting a generic
signature for the parent, see scala#6349.

doc-tool/src/dotty/tools/dottydoc/staticsite/Yaml.java

@@ -0,0 +1,22 @@
+package dotty.tools.dottydoc.staticsite;
+
+import com.fasterxml.jackson.dataformat.yaml.YAMLFactory;
+import com.fasterxml.jackson.databind.ObjectMapper;
+import com.fasterxml.jackson.core.type.TypeReference;
+
+import java.util.HashMap;
+import java.io.ByteArrayInputStream;
+
+public class Yaml {
+
+  public static HashMap<String, Object> apply(String input)
+      throws java.io.UnsupportedEncodingException, java.io.IOException {
+    ByteArrayInputStream is = new ByteArrayInputStream(input.getBytes("UTF-8"));
+    ObjectMapper mapper = new ObjectMapper(new YAMLFactory());
+
+    TypeReference<HashMap<String, Object>> typeRef =
+        new TypeReference<HashMap<String, Object>>() {};
+
+    return mapper.readValue(is, typeRef);
+  }
+}