sagemathgh-40857: Fix pdf docbuild failure if destination file exists
    
The destination of shutil.move must be the filename and not a directory
to take advantage of move-overwrite filesystem semantic.

Fixes sagemath#40854
    
URL: sagemath#40857
Reported by: Volker Braun
Reviewer(s):

Author: Release Manager

src/sage_docbuild/builders.py

@@ -296,9 +296,10 @@ def pdf(self):
         # Move generated PDFs
         for pdf in tex_dir.glob("*.pdf"):
             try:
-                shutil.move(str(pdf), pdf_dir)
+                dst_pdf = os.path.join(pdf_dir, os.path.basename(pdf))
+                shutil.move(str(pdf), dst_pdf)
             except Exception as e:
-                logger.error(f"Failed moving {pdf} to {pdf_dir}: {e}")
+                logger.error(f"Failed moving {pdf} to {dst_pdf}: {e}")
                 raise
 
         logger.info(f"Build finished. The built documents can be found in {pdf_dir}.")