feat: pr review changes

Author: ricardohsmello

docs/50-aggregation/4-group.mdx

@@ -194,6 +194,8 @@ GROUP BY year;
     <TabItem value="java" label="Java">
       <div>
         ```java
+        MongoCollection<Document> reviews = library.getCollection("reviews");
+
        	reviews.aggregate(
             List.of(
             Aggregates.group("$bookId",
@@ -339,7 +341,9 @@ GROUP BY year;
           <div>
             ```java
             import static com.mongodb.client.model.Sorts.descending;
-            
+
+            MongoCollection<Document> reviews = library.getCollection("reviews");
+
             reviews.aggregate(
                 List.of(Aggregates.group(
                     "$name",
@@ -353,6 +357,8 @@ GROUP BY year;
         <TabItem value="$sortByCount" label="$sortByCount">
           <div>
             ```java
+            MongoCollection<Document> reviews = library.getCollection("reviews");
+            
             reviews.aggregate(
                 List.of(
                     Aggregates.sortByCount("$name"))

docs/50-aggregation/5-lookup.mdx

@@ -170,6 +170,8 @@ The $lookup operation creates an array within each book document. Using $unwind
         ```java
         import com.mongodb.client.model.Aggregates;
 
+        MongoCollection<Document> books = library.getCollection("books");
+
         books.aggregate(
           List.of(
             Aggregates.lookup(