[Doc][R] Document GCSFileSystem for R package

[asfimport]

We should update the cloud storage vignette and the filesystem RD to show configuration and usage of GCSFileSystem.

Reporter: Will Jones / @wjones127
Assignee: Will Jones / @wjones127

Related issues:

• [R] Add bindings for GCS filesystem (is related to)

PRs and other links:

• GitHub Pull Request #13601

_{Note: This issue was originally created as ARROW-16887. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 13601
#13601