Add annotations to Dataset in example

[tsalo]

A colleague noticed that the examples in the documentation that create annotations (e.g., https://nimare.readthedocs.io/en/stable/auto_examples/03_annotation/01_plot_tfidf.html#sphx-glr-auto-examples-03-annotation-01-plot-tfidf-py and https://nimare.readthedocs.io/en/stable/auto_examples/03_annotation/02_plot_cognitive_atlas.html#sphx-glr-auto-examples-03-annotation-02-plot-cognitive-atlas-py) don't show how to add those annotations back into the Dataset object.

[tsalo]

I think it's just a matter of having something like dset.annotations = df in the examples, but if folks want to add to their annotations, rather than replace them, it will require a bit more code.