Improve organization of “chart gallery”

[maggiejaenicke]

There is a carousel for the 2023 30-day chart challenge, but other charts are organized sort of randomly. As this section grows, we’ll want to move them all to carousels so only a couple appear at once, but all can be browsed.

• Organize U.S. River Conditions

  • Released quarterly
  • Ideally the most recent one or two would appear and the rest could be found by navigating a carousel
  • PR adding the most recent river conditions

• Flow Tiles

  • Put in a carousel

Notes:
-Relates to / replaces #39
-There's a csv that controls everything on the landing page

Assignee: Maggie Jaenicke
(putting this here since I don't have repo access)