docs: fix two broken docs-site links in the README

[powersemmi]

Description

Two docs-site links in the README 404: the quickstart link under Run it and the testing-guide link under Testing the service both pointed at subpaths without the version segment. The site is published with mike, where latest is a real directory, so subpaths only resolve under /latest/ (the root serves the latest index, but subpaths are not aliased at the root). The rest of the README already uses the /latest/ form for its docs links; these two were missed.

Repoint both to the /latest/ form:

• …/ruststream/getting-started/quickstart/ -> …/ruststream/latest/getting-started/quickstart/
• …/ruststream/guides/testing/ -> …/ruststream/latest/guides/testing/

Both now return 200.

Docs-only; no code change.

Type of change

• Documentation (typos, code examples, or any documentation updates)
• Bug fix (a non-breaking change that resolves an issue)
• New feature (a non-breaking change that adds functionality)
• Breaking change (removing or renaming public API, changing a signature, tightening bounds, or
  raising the MSRV - a minor version bump pre-1.0)
• This change requires a documentation update

Checklist

• My code follows the project's style guidelines (just check passes: rustfmt, clippy, and
  cargo check with all features and with --no-default-features)
• I have performed a self-review of my own code
• I have made the necessary changes to the documentation (rustdoc and/or the docs site)
• My changes generate no new warnings (clippy runs with -D warnings)
• I have added tests that validate my fix or new feature
• New and existing tests pass locally (just test)
• Public items carry a compiling # Examples doctest, and user-facing changes are reflected in
  examples/ where applicable