Docs: validating/parsing optional params

## Status
Largely supported on `main` — this is now a **documentation task**.

Optional params with a constrained value set can already be expressed with typed generics in the path, e.g. `/:lang?`, which both narrows the type (`lang?: 'en' | 'ru'`) and drives runtime parsing (`packages/paths`). Segments that don't match fall through instead of being accepted as any value.

## What's left
- Document how to validate/parse optional params via typed generics (`:lang?`) in the paths / create-route docs.
- Verify and document the fall-through-to-`notFound` behavior with `lang === undefined`.
- If a real gap is found while writing docs, reopen with a concrete failing case.

## Traceability
Original request: atomic-router/atomic-router#63 — https://github.com/atomic-router/atomic-router/issues/63