docs: refine routing/app structure and runtime-route guidance

Follow-up to the routing docs restructure:

Routing guide (docs/guide/routing.md):
- Add an example that registers multiple handlers in a single app.Get call
  (route-specific middleware before the business handler).
- Correct the runtime-routes note: dynamic add/remove IS supported (via
  RebuildTree and the RemoveRoute* methods) but not recommended; link to them.
- Drop the clunky "Examples of route paths based on strings" subheading and
  keep the route-declaration-order rule prominent (no longer Express-led).

App reference (docs/api/app.md):
- Sort State / SharedState alphabetically (now between SetTLSHandler and Test).
- Group RebuildTree and the RemoveRoute* methods under a new "Route Management"
  section.
- Move the template Helpers section to the bottom of the page.

Grouping guide (docs/guide/grouping.md):
- Add Route and RouteChain sections so the dedicated grouping guide covers the
  same helpers shown in the routing guide.

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