Add summary info at top of each notebook

[tomdonaldson]

Add brief sections to the top of each notebook. Adapt examples below as appropriate.

In https://numpy.org/numpy-tutorials/content/mooreslaw-tutorial.html

• What you'll do
• Skills you'll learn

or in https://github.com/fornax-navo/fornax-demo-notebooks/blob/main/forced_photometry/multiband_photometry.ipynb:

• Learning Goals
• Introduction
• ...