docs: flatten routing examples into tabs and fix review findings

- routing.md: convert the "Get vs Use vs All" and "Parameters" example
  sections to <Tabs> (matching the Constraints section) so the stacked code
  blocks are flattened and grouped by use case.
- routing.md: the multiple-handler example now reads back the value it stashes
  via c.Locals and shows the 401-vs-200 outcomes; clarify that a handler
  returning without c.Next() stops the chain.
- routing.md: scope the Express comparison for route order to registration
  order only (method helpers match the exact path).
- grouping.md: add the note that a chained All is prefix-matched middleware
  (like app.Use), not the exact-path App.All.
- use.md: replace the non-existent random.String(32) with a literal value.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>