Add example for SolrDocument + Mapstruct

まだ誰も着手していません。

It would be nice to have an example that shows how to use SolrDocument with Mapstruct.

I was trying to use MapStruct with SolrDocument and couldn't find a comprehensive example that could guide me . I was facing below issue during compiling:
**Can't generate mapping method from iterable type to non-iterable type.**
Reason is [SolrDocument](https://github.com/apache/lucene-solr/blob/master/solr/solrj/src/java/org/apache/solr/common/SolrDocument.java) implements Iterable interface and Target DTOs doesn't .
So i came up with below solution to deal with this.
**Source Class**
[SolrDocument](https://github.com/apache/lucene-solr/blob/master/solr/solrj/src/java/org/apache/solr/common/SolrDocument.java)
Here SolrDocument internally has Map _fields;

**Destination Class**:
`public class ItemDTO {

private String id;
private String displayName;
private String availability;
private String price;

//getter
//setter
}
}`

**Error : Can't generate mapping method from iterable type to non-iterable type.**

**Solution**:
Created SolrResponseWrapper around SolrDocument like below:
`public class SolrResponseWrapper {

private SolrDocument document;

public SolrResponseWrapper(SolrDocument document) {
this.document = document;
}

public String get(String key) {
return this.document.get(key).toString();
}
}`

**Now Mapper is defined below for SolResponseWrapper to ItemDTO.**

`@Mapper
public interface ItemMapper {
ItemMapper INSTANCE = Mappers.getMapper(ItemMapper);

@Mappings({
@Mapping(expression = "java(record.get(\"product_id\"))", target = "id"),
@Mapping(expression = "java(record.get(\"display_name\"))", target = "displayName"),
@Mapping(expression = "java(record.get(\"availability\"))", target = "availability"),
@Mapping(expression = "java(record.get(\"price\"))", target = "price")
})
ItemDTO convertToItemDTO(SolrResponseWrapper record);
}`

If you think this is a good addition, please let me know. I will raise PR to have it included as an example.

まず、リポジトリ内の既存の Java サンプルレイアウトを確認し、次に issue にある SolrDocument、SolrResponseWrapper、ItemDTO、ItemMapper の例を出発点として使用します。wrapper ベースのマッピングを示す、焦点を絞った例を追加し、その例が報告された iterable-to-non-iterable の問題とその解決策を実証していることを確認します。

索引モデルが issue の本文から書いたものです。