docs: broaden README scope beyond connection lifecycle

The opening tagline still described go-nuts as connection-lifecycle-only (connect/drain/shutdown), stale since the natsmsg/jsconsumer/backoff subpackages landed — half the library's value read as an afterthought at the bottom. Lead with the full scope, retitle the root-package section so the subpackages are clearly the rest rather than a footnote, and drop the internal ticket ref from the front page now that the repo is public.