Add ClickPipes migration path for BigQuery + restructure BigQuery migration section

docs/_snippets/clickpipes/bigquery/_permissions.md

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+import cp_iam from '@site/static/images/integrations/data-ingestion/clickpipes/bigquery/cp_iam.png';
+import Image from '@theme/IdealImage';
+
+ClickPipes authenticates to your Google Cloud project using a [service account key](https://docs.cloud.google.com/iam/docs/keys-create-delete). We recommend creating a dedicated service account with the minimum required set of [permissions](#permissions) to allow ClickPipes to export data from BigQuery, load it into the staging GCS bucket, and read it into ClickHouse.
+
+To create a service account:
+
+1. Select **IAM and admin** from the navigation menu in the Google Cloud console
+2. Select **Service accounts**
+3. Click **Create service account**
+
+<Image img={cp_iam} alt="Creating a service account key with BigQuery and Cloud Storage permissions" size="lg" border/>
+
+### Service account permissions {#permissions}
+
+The following service account permissions are required:
+
+#### BigQuery {#bigquery}
+
+The service account must have the following BigQuery roles:
+
+* [`roles/bigquery.dataViewer`](https://docs.cloud.google.com/bigquery/docs/access-control#bigquery.dataViewer)
+* [`roles/bigquery.jobUser`](https://docs.cloud.google.com/bigquery/docs/access-control#bigquery.jobUser)
+
+To further scope access, we recommend using [IAM conditions](https://docs.cloud.google.com/bigquery/docs/conditions) to restrict the resources the role has access to. For example, you can restrict the `dataViewer` role to the specific dataset containing the tables you want to sync:
+
+```plaintext
+resource.name.startsWith("projects/<PROJECT_ID>/datasets/<DATASET_NAME>")
+```
+
+#### Cloud Storage {#cloud-storage}
+
+The service account must have the following Cloud Storage roles:
+
+* [`roles/storage.objectAdmin`](https://docs.cloud.google.com/storage/docs/access-control/iam-roles#storage.objectAdmin)
+* [`roles/storage.bucketViewer`](https://docs.cloud.google.com/storage/docs/access-control/iam-roles#storage.bucketViewer)
+
+To further scope access, we recommend using [IAM conditions](https://docs.cloud.google.com/bigquery/docs/conditions) to restrict the resources the role has access to. For example, you can restrict the `objectAdmin` and `bucketViewer` roles to the dedicated bucket created for ClickPipes syncs.
+
+```plaintext
+resource.name.startsWith("projects/_/buckets/<BUCKET_NAME>")
+```

docs/_snippets/clickpipes/bigquery/_select-data-source.md

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+import cp_step0 from '@site/static/images/integrations/data-ingestion/clickpipes/cp_step0.png';
+import cp_step1 from '@site/static/images/integrations/data-ingestion/clickpipes/bigquery/cp_step1.png';
+import Image from '@theme/IdealImage';
+
+**1.** In ClickHouse Cloud, select **Data sources** in the main navigation menu and click **Create ClickPipe**.
+
+    <Image img={cp_step0} alt="Select imports" size="lg" border/>
+
+**2.** Click the **BigQuery** tile.
+
+    <Image img={cp_step1} alt="Select BigQuery tile" size="lg" border/>

docs/_snippets/clickpipes/bigquery/_service-account-key.md

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+Depending on the migration approach you plan to use, you will need to create one of the following credentials:
+
+| Migration approach     | Credential type | Documentation                                                                                                                                                              | Notes                                                                          |
+|------------------------|-----------------|----------------------------------------------------------------------------------------------------------------------------------------------------------------------------|--------------------------------------------------------------------------------|
+| ClickPipes for CDC     | Service key     | [Create a service key](https://docs.cloud.google.com/iam/docs/keys-create-delete#iam-service-account-keys-create-console)                                                  | Make sure to select **JSON** rather than **P12** as the **Key type**.          |
+| GCS bulk-load          | `HMAC` key        | [Create an `HMAC` key](https://docs.cloud.google.com/storage/docs/authentication/managing-hmackeys)                                                                          | -                                                                              |

docs/_snippets/clickpipes/bigquery/_setup-connection.md

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+import cp_step2 from '@site/static/images/integrations/data-ingestion/clickpipes/bigquery/cp_step2.png';
+import Image from '@theme/IdealImage';
+
+To set up a new ClickPipe, you must provide details on how to connect to and authenticate with your BigQuery data warehouse, and a staging GCS bucket.
+
+**1.** Upload the `.json` key for the service account you created for ClickPipes. Ensure the service account has the minimum required set of [permissions](/integrations/clickpipes/bigquery/overview#permissions).
+
+    <Image img={cp_step2} alt="Upload service account key" size="lg" border/>
+
+**2.** Select the **Replication method**. In Private Preview, the only supported option is [**Initial load only**](/integrations/clickpipes/bigquery/overview#initial-load).
+
+**3.** Provide the path to the GCS bucket for staging data during the initial load.
+
+**4.** Click **Next** to validate.