PR summary

Motivated by wanting to position an image relative to text and being confused, I reworked the AnnotationBBox demo:

• emphasize that this is a method for using artists as annotations, because that wasn't clear to me when I was looking through the docs
• broke up the demo into separate sections (text, image, arbitrary artist based on the different offset box classes) because I was having trouble keeping my place and mapping code to image when I was using this example to make something.

Also added AnnotationBbox to the Annotation guide because it's currently not there.

Current docs:

• https://matplotlib.org/stable/users/explain/text/annotations.html#advanced-annotation
• https://matplotlib.org/stable/gallery/text_labels_and_annotations/demo_annotation_box.html

PR checklist

• "closes #0000" is in the body of the PR description to link the related issue
• new and changed code is tested
• Plotting related features are demonstrated in an example
• New Features and API Changes are noted with a directive and release note
• Documentation complies with general and docstring guidelines