Improve lossless compression example (#2488)

Simplify the previous version and remove the side effect of changing the PDF in ways not just related to lossless compression (e.g. links on content page did not work in the created PDF).

docs/user/file-size.md

@@ -81,16 +81,11 @@ Deflate compression can be applied to a page via
 [`page.compress_content_streams`](https://pypdf.readthedocs.io/en/latest/modules/PageObject.html#pypdf._page.PageObject.compress_content_streams):
 
 ```python
-from pypdf import PdfReader, PdfWriter
-
-reader = PdfReader("example.pdf")
-writer = PdfWriter()
+from pypdf import PdfWriter
 
-for page in reader.pages:
- writer.add_page(page)
+writer = PdfWriter(clone_from="example.pdf")
 
 for page in writer.pages:
- # ⚠️ This has to be done on the writer, not the reader!
  page.compress_content_streams() # This is CPU intensive!
 
 with open("out.pdf", "wb") as f: