Standardize name for "splat" URL elements?

[genehack]

I wonder if another name would be more informative here. Maybe staging/[[...slug]]/… to match Next.js docs? Same would go for pathogens/[[...slug]]. blog/[id] is meaningful as-is.

  if (params && params["slug"]) {
    notFound();
  }

Not the fault of this PR since the old implementation uses [[...staging]]

Originally posted by @victorlin in https://github.com/nextstrain/nextstrain.org/pull/1131#discussion_r2008485439