docs: complete docstring for DocumentJoiner code example (#8593)

* initial import

* changing a method to static

* reverting staticmethod

Author: davidsbatista

Diff for: haystack/components/joiners/document_joiner.py

@@ -54,7 +54,20 @@ class DocumentJoiner:
     ### Usage example:
 
     ```python
+    from haystack import Pipeline, Document
+    from haystack.components.embedders import SentenceTransformersTextEmbedder, SentenceTransformersDocumentEmbedder
+    from haystack.components.joiners import DocumentJoiner
+    from haystack.components.retrievers import InMemoryBM25Retriever
+    from haystack.components.retrievers import InMemoryEmbeddingRetriever
+    from haystack.document_stores.in_memory import InMemoryDocumentStore
+
     document_store = InMemoryDocumentStore()
+    docs = [Document(content="Paris"), Document(content="Berlin"), Document(content="London")]
+    embedder = SentenceTransformersDocumentEmbedder(model="sentence-transformers/all-MiniLM-L6-v2")
+    embedder.warm_up()
+    docs_embeddings = embedder.run(docs)
+    document_store.write_documents(docs_embeddings['documents'])
+
     p = Pipeline()
     p.add_component(instance=InMemoryBM25Retriever(document_store=document_store), name="bm25_retriever")
     p.add_component(
@@ -67,7 +80,7 @@ class DocumentJoiner:
     p.connect("embedding_retriever", "joiner")
     p.connect("text_embedder", "embedding_retriever")
     query = "What is the capital of France?"
-    p.run(data={"query": query})
+    p.run(data={"query": query, "text": query, "top_k": 1})
     ```
     """