docs: introduce GCS backend docs

docs/integrations/object-storage/gcs.md

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+# GCS Storage Backend
+
+`delta-rs` offers native support for using Google Cloud Storage (GCS) as an object storage backend.
+
+You don’t need to install any extra dependencies to read/write Delta tables to GCS with engines that use `delta-rs`. You do need to configure your GCS access credentials correctly.
+
+## Using Application Default Credentials
+
+Application Default Credentials (ADC) is a strategy used by GCS to automatically find credentials based on the application environment.
+
+If you are working from your local machine and have ADC set up then you can read/write Delta tables from GCS directly, without having to pass your credentials explicitly.
+
+## Example: Write Delta tables to GCS with Polars
+
+Using Polars, you can write a Delta table to GCS like this:
+
+```python
+# create a toy dataframe
+import polars as pl
+df = pl.DataFrame({"foo": [1, 2, 3, 4, 5]})
+
+# define path
+table_path = "gs://bucket/delta-table"
+
+# write Delta to GCS
+df.write_delta(table_path)
+```
+
+## Passing GCS Credentials explicitly
+
+Alternatively, you can pass GCS credentials to your query engine explicitly.
+
+For Polars, you would do this using the `storage_options` keyword. This will forward your credentials to the `object store` library that Polars uses under the hood. Read the [Polars documentation](https://docs.pola.rs/api/python/stable/reference/api/polars.DataFrame.write_delta.html) and the [`object store` documentation](https://docs.rs/object_store/latest/object_store/gcp/enum.GoogleConfigKey.html#variants) for more information.
+
+## Delta Lake on GCS: Required permissions
+
+You will need the following permissions in your GCS account:
+
+- `storage.objects.create`
+- `storage.objects.delete` (only required for uploads that overwrite an existing object)
+- `storage.objects.get` (only required if you plan on using the Google Cloud CLI)
+- `storage.objects.list` (only required if you plan on using the Google Cloud CLI)
+
+For more information, see the [GCP documentation](https://cloud.google.com/storage/docs/uploading-objects)