Implement is_available for DOIDownloader

[K-Meech]

Description of the desired feature:
Currently, using Pooch.is_available with a DOIdownloader (e.g. for zenodo) throws:

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\kimme\miniforge3\envs\zarr-benchmarks-p\Lib\site-packages\pooch\core.py", line 739, in is_available
    raise NotImplementedError(error_msg) from error
NotImplementedError: Downloader '<pooch.downloaders.DOIDownloader object at 0x000001B32B705810>' does not support availability checks.

Would it be possible to implement the is_available check for DOIDownloaders?

Are you willing to help implement and maintain this feature?
I'd be happy to draft a PR adding this feature, if you think it would make a good addition.