Explicit fields in Corpus admin

Ref. https://gitlab.teklia.com/arkindex/enterprise/-/issues/43#note_272548

arkindex/documents/admin.py

@@ -31,6 +31,8 @@ class ElementTypeInline(admin.TabularInline):
 
 
 class CorpusAdmin(admin.ModelAdmin):
+    fields = ("id", "name", "description", "top_level_type", "public", "indexable", "maximum_task_ttl", "created")
+    readonly_fields = ("id", "created")
     list_display = ("id", "name", "public", "top_level_type", "created")
     search_fields = ("name", )
     inlines = (ElementTypeInline, )