✨(backend) annotate number of accesses on documents

The new UI will display the number of accesses on each document.

/!\ Once team accesses will be used, this will not represent the number
    of people with access anymore and will have to be improved by
    computing the number of people in each team.

src/backend/core/api/serializers.py

@@ -141,6 +141,7 @@ class ListDocumentSerializer(BaseResourceSerializer):
     """Serialize documents with limited fields for display in lists."""
 
     is_favorite = serializers.BooleanField(read_only=True)
+    nb_accesses = serializers.IntegerField(read_only=True)
 
     class Meta:
         model = models.Document
@@ -152,6 +153,7 @@ class ListDocumentSerializer(BaseResourceSerializer):
             "is_favorite",
             "link_role",
             "link_reach",
+            "nb_accesses",
             "title",
             "updated_at",
         ]
@@ -162,6 +164,7 @@ class ListDocumentSerializer(BaseResourceSerializer):
             "is_favorite",
             "link_role",
             "link_reach",
+            "nb_accesses",
             "updated_at",
         ]
 
@@ -181,6 +184,7 @@ class DocumentSerializer(ListDocumentSerializer):
             "is_favorite",
             "link_role",
             "link_reach",
+            "nb_accesses",
             "title",
             "updated_at",
         ]
@@ -191,6 +195,7 @@ class DocumentSerializer(ListDocumentSerializer):
             "is_avorite",
             "link_role",
             "link_reach",
+            "nb_accesses",
             "updated_at",
         ]