Travel photos for /footprint/

One subfolder per park. A folder for every US national park already exists here (named with the park slug, e.g. big-bend, grand-canyon); create new folders for state parks as you visit them (lowercase, hyphenated, e.g. enchanted-rock).

To show a park on the page:

1. Drop photos into its folder (jpg, jpeg, png, webp, or gif). Resize to roughly 1600px on the long edge first so the repo and page stay light. Photos sort by filename, so prefix numbers to control the order (01-rim.jpg, 02-sunset.jpg).
2. Add the park to _data/travel.yml with a slug matching the folder name. Only parks listed there appear on the page; empty folders are ignored.

The .gitkeep files only keep the empty folders in git; delete one when its folder gets real photos, or leave it, either is fine.