docs: Add docs for gcs backend

[ClSlaid]

Summary

1. add documentation for the GCS backend
2. add a runnable example for the GCS backend

Signed-off-by: ClSlaid [email protected]

I hereby agree to the terms of the CLA available at: https://databend.rs/dev/policies/cla/

[Xuanwo]

Please also add docs in lib.rs and gcs mod.

For example: https://github.com/datafuselabs/opendal/blob/d6cbd898ae2826d5b2327b90a577b6b33b6680da/src/services/s3/mod.rs#L15

[ClSlaid]

/cc @Xuanwo PTAL