Update penguins doc

[jlmelville]

Striking while the doc-build iron is hot, this PR:

• updates some URLs that had got stale. In particular, the URL for the penguin image, but also the CSV file itself.
• the URLs are now github permalinks, so hopefully even if future breaking changes occur, these particularly URLs will stay valid.
• updates to the penguin dataset meant that:
  • there are now a different number of non-NA rows.
  • also the column names had changed slightly.
  • and there was a year column added.
• the text has been adjusted to reflect the above and I drop the year column in the pairplot.
  • I had to regenerate the pairplot. It has larger dimensions (not sure how to change that via jupyter lab) but is otherwise quite similar.
• also to be on the safe side I regenerated the UMAP plot (it doesn't change much, reassuringly).
• one change to the digits example: there was a minor change to the PIL API.

[jlmelville]

how did I break all the builds with this change? 🤔

[lmcinnes]

The breakage is, I think, my fault. Don't worry about it for now.