DOC: Simplify PaperSize examples (py-pdf#2504)

Author: j-t-1

docs/modules/PaperSize.rst

@@ -11,25 +11,21 @@ Add blank page with PaperSize
 .. code-block:: python
     :linenos:
 
-    from pypdf import PaperSize, PdfReader, PdfWriter
+    from pypdf import PaperSize, PdfWriter
 
-    pdf_reader = PdfReader("sample.pdf")
-    pdf_writer = PdfWriter()
-    pdf_writer.append_pages_from_reader(pdf_reader)
-    pdf_writer.add_blank_page(PaperSize.A8.width, PaperSize.A8.height)
+    writer = PdfWriter(clone_from="sample.pdf")
+    writer.add_blank_page(PaperSize.A8.width, PaperSize.A8.height)
     with open("output.pdf", "wb") as output_stream:
-        pdf_writer.write(output_stream)
+        writer.write(output_stream)
 
 Insert blank page with PaperSize
 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
 .. code-block:: python
     :linenos:
 
-    from pypdf import PaperSize, PdfReader, PdfWriter
+    from pypdf import PaperSize, PdfWriter
 
-    pdf_reader = PdfReader("sample.pdf")
-    pdf_writer = PdfWriter()
-    pdf_writer.append_pages_from_reader(pdf_reader)
-    pdf_writer.insert_blank_page(PaperSize.A8.width, PaperSize.A8.height, 1)
+    writer = PdfWriter(clone_from="sample.pdf")
+    writer.insert_blank_page(PaperSize.A8.width, PaperSize.A8.height, 1)
     with open("output.pdf", "wb") as output_stream:
-        pdf_writer.write(output_stream)
+        writer.write(output_stream)