docs: def get_materialized_data_cache(doc=None):
      """Return the cache directory where data can be written during a build, usually for
      a Jupyter notebook that generates many files for each execution"""
  
      from metapack.constants import MATERIALIZED_DATA_PREFIX
      from os.path import join
  
      if not doc:
          from metapack import Downloader
          downloader = Downloader()
          return downloader.cache.getsyspath(MATERIALIZED_DATA_PREFIX)
      else:
          dr = doc._cache.getsyspath(join(MATERIALIZED_DATA_PREFIX, doc.name))
  
          ensure_dir(dr)
  
          return dr