Docs/adapters review (#92223)
Some corrections to the adapters docs.
- The example deploy script ran `cat .next/BUILD_ID` before `pnpm
build`. Under `set -euo pipefail` that kills the script immediately
since the file doesn't exist yet. Moved the build step first.
- `resolveRoutes` requires `URL` and `Headers` objects, not strings. The
example would cause TypeScript/runtime errors.
- Also documented the 6 return fields ~ other adapters might need to use
all of them for redirects, external rewrites, and middleware
short-circuiting.
