(docs/test): use cross-fetch instead of isomorphic-fetch

[agilgur5]

• Per the cross-fetch FAQ (https://github.com/lquixada/cross-fetch#why-not-isomorphic-fetch), isomorphic-fetch has issues with RN, hasn't been maintained in years, and has outdated dependencies
  • cross-fetch hasn't been maintained in some months now too though, but only a few unresolved issues/PRs