Improve the media includes

* [ ] 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 `` 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).

Start by inventorying the gallery, icon, figure/start, figure/panel, figure/end, img, video, and thumbnail includes, then review their uses alongside /editing/images and /editing/videos. Done means the media includes are consistently organized, simple captions remain supported, complex multi-panel figures are possible, and the editing pages are consolidated with large-resource guidance.

Written by the indexing model from the issue text.