docs(multi-step): point at the task-level trajectory.json convention
Reduced to a single cross-reference. Everything else this PR carried is already on main.
#2529 merged, and #2632 then added a task-level example task plus the task-level section of run-jobs/load-trajectory.mdx — which is what this PR was for. So the example task and the load-trajectory docs here were duplicates and are dropped.
What is left is the one gap #2632 did not cover: docs/content/docs/tasks/multi-step.mdx documents --resume-trajectory and --load-trajectory but never mentions the per-step trajectory.json, so a reader on that page has no way to learn that steps/<first-step>/trajectory.json exists. This adds a two-line pointer to the task-level section and states that only the first step's file is read — a later step's is ignored.
Two lines, one file. Close it instead if you'd rather the cross-reference not exist.