Adding logic to remove temporary files after CV has been generated

[pgoslatara]

At the moment the generate_cv.py script initially removes non-.gitkeep files from the clean_pages and clean_images directories. At the end of this script these files remain in the filesystem. This is kinda messy, especially if we scale out in future by generating multiple CVs at a time.

This PR moves the cleanup_files function to the end of the script so the filesystem is not polluted after successful runs. The pathlib library is used to create the cv_pages directory at the start of the script, this does not fail if the directory already exists so an unsuccessful run which leaves a cv_pages directory will not block future runs.

Both pathlib and shutil are internal libraries so they will be available on all python runtimes.