Split the monstrous C API manual files in smaller parts.
diff --git a/Doc/c-api/index.rst b/Doc/c-api/index.rst
index 086b880..12a1ec7 100644
--- a/Doc/c-api/index.rst
+++ b/Doc/c-api/index.rst
@@ -24,4 +24,4 @@
    concrete.rst
    init.rst
    memory.rst
-   newtypes.rst
+   objimpl.rst