Add new example notebook for transfer learning

[joelnkn]

Adding notebooks following the checklist in #772 (and #556 / #676).

[kevingreenman]

@joelnkn thanks for opening the draft PR! I suggest we do one PR per notebook to keep things moving, so you can open separate PRs for the active learning and other future notebooks.

Could you make a new branch with a descriptive name (e.g. transfer-learning-notebook) on your personal fork, and make the PR from that instead of from the main branch of your fork? You might have to close this PR and open a new one, or maybe there's a way to just change the source branch. This will make it easier for you since you might be working on multiple new branches at the same time for different notebooks to add. LMK if you have any questions.

[kevingreenman]

Also, before we mark this or future notebook PRs as ready for review, you'll need to do two things each time you add a notebook:

1. Add a *.nblink file with appropriate name and contents in docs/source/. This enables us to keep the notebooks in the top-level examples/ directory where they're easily findable by users, while also making them available to display as part of the documentation.
2. Add the notebook name from the *.nblink file to the docs/source/notebooks.rst file so that the new notebook appears in the sidebar of the documentation.