Add more context to example notebooks

[wkearn]

The example notebooks in #33 don't have a lot of information about what they are demonstrating. I imagine some of these notebooks might get incorporated into #32, but any that remain on their own could do with a bit more text, more descriptive titles and thumbnails, etc.

[wkearn]

This was largely handled by #102, which moved a lot of the content in the example notebooks to a "Getting Started" tutorial. We still have separate example notebooks on plotting, GridObject algebra and excess topography that could use some updating.