docs: restructure routing guide and split handler partials

Reorganize the routing guide so route-handler registration and the HTTP
request methods come first and the special handler shapes come last, and
split the shared handler partial so the guide and the App API reference can
reuse it cleanly.

Guide (docs/guide/routing.md):
- New section order: Route Handlers (with a runnable minimal app) -> Get vs
  Use vs All -> Paths -> Parameters/Constraints -> Middleware/Use -> Grouping
  (incl. Route and RouteChain) -> Automatic HEAD routes -> Handler types.
- Add a Get vs Use vs All comparison (exact vs prefix matching, all-methods
  vs single method, empty Use, ordered middleware chain) plus a 404-vs-405
  note.
- Document Route and RouteChain and a "pick the helper" decision guide.
- Clarify multiple-parameters-per-segment and the *1/*2 and +1/+2 positional
  retrieval; clarify the regex-route vs regex-constraint distinction; fix the
  float constraint example and a missing parenthesis.

Partials (docs/partials/routing/):
- Split handler.md into a heading-neutral core (signatures + basic examples),
  a new use.md (Use), and a new handler-types.md (Express/net-http/fasthttp
  adapter shapes), so each host page supplies its own heading levels.

App reference (docs/api/app.md):
- Render the Use partial at the correct heading level and move State /
  SharedState out of the Routing section.
- Set toc_max_heading_level to 4 so DomainParam appears in the sidebar.
- Fix the Stack example imports and two inline-code rendering bugs.

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