✨(backend) add "tree" action on document API endpoint

We want to display the tree structure to which a document belongs
on the left side panel of its detail view. For this, we need an
endpoint to retrieve the list view of the document's ancestors
opened.

By opened, we mean that when display the document, we also need to
display its siblings. When displaying the parent of the current
document, we also need to display the siblings of the parent...

src/backend/core/tests/documents/test_api_documents_children_list.py

@@ -15,7 +15,7 @@ pytestmark = pytest.mark.django_db
 
 
 def test_api_documents_children_list_anonymous_public_standalone():
-    """Anonymous users should be allowed to retrieve the children of a public documents."""
+    """Anonymous users should be allowed to retrieve the children of a public document."""
     document = factories.DocumentFactory(link_reach="public")
     child1, child2 = factories.DocumentFactory.create_batch(2, parent=document)
     factories.UserDocumentAccessFactory(document=child1)

src/backend/core/tests/documents/test_api_documents_retrieve.py

@@ -44,6 +44,7 @@ def test_api_documents_retrieve_anonymous_public_standalone():
             "partial_update": document.link_role == "editor",
             "restore": False,
             "retrieve": True,
+            "tree": True,
             "update": document.link_role == "editor",
             "versions_destroy": False,
             "versions_list": False,
@@ -100,6 +101,7 @@ def test_api_documents_retrieve_anonymous_public_parent():
             "partial_update": grand_parent.link_role == "editor",
             "restore": False,
             "retrieve": True,
+            "tree": True,
             "update": grand_parent.link_role == "editor",
             "versions_destroy": False,
             "versions_list": False,
@@ -189,6 +191,7 @@ def test_api_documents_retrieve_authenticated_unrelated_public_or_authenticated(
             "partial_update": document.link_role == "editor",
             "restore": False,
             "retrieve": True,
+            "tree": True,
             "update": document.link_role == "editor",
             "versions_destroy": False,
             "versions_list": False,
@@ -252,6 +255,7 @@ def test_api_documents_retrieve_authenticated_public_or_authenticated_parent(rea
             "partial_update": grand_parent.link_role == "editor",
             "restore": False,
             "retrieve": True,
+            "tree": True,
             "update": grand_parent.link_role == "editor",
             "versions_destroy": False,
             "versions_list": False,
@@ -424,6 +428,7 @@ def test_api_documents_retrieve_authenticated_related_parent():
             "partial_update": access.role != "reader",
             "restore": access.role == "owner",
             "retrieve": True,
+            "tree": True,
             "update": access.role != "reader",
             "versions_destroy": access.role in ["administrator", "owner"],
             "versions_list": True,

src/backend/core/tests/documents/test_api_documents_trashbin.py

@@ -87,6 +87,7 @@ def test_api_documents_trashbin_format():
             "partial_update": True,
             "restore": True,
             "retrieve": True,
+            "tree": True,
             "update": True,
             "versions_destroy": True,
             "versions_list": True,