Improve the media includes

[ctrueden]

• The gallery include needs a cleanup pass.
• The icon include could be generalized.

• Finish figure / panel includes (see also next item).

• The img and video includes both use <figure> sometimes, but we should reconcile this with the dedicated figure/* includes, that allow for more complex figures e.g. multi-panel. Goal: make the simple things easy (i.e. still support caption= directly on img and video), but the more complex things possible/elegant (i.e. multi-panel figures with figure/start, figure/panel, and figure/end includes nicely organized).
• The thumbnail include needs review in conjunction with gallery as well as its direct uses.
• Combine /editing/images and /editing/videos into /editing/media, add section about large resources (recommending Zenodo or FigShare).