Document Redshift COPY IAM role for reading exposure data from S3

[petzel]

What does this PR do? What is the motivation?

Fixes DOCS-XXXXX

Documents how to set up the IAM role that Redshift assumes to read exposure data from S3 during warehouse-native experiment analysis. The Redshift warehouse connection now requires a Copy IAM role ARN instead of embedding raw credentials in the COPY query.

Changes to content/en/experiments/guide/connecting_a_data_warehouse.mdoc.md (Redshift section):

• Adds a Step 2 subsection covering: creating the read-only S3 IAM policy, creating a role with a Redshift trust policy, attaching the policy, and associating the role with the cluster (provisioned and Serverless).
• Adds the new Copy IAM role ARN field to the Step 3 connection settings.

Merge instructions

Merge readiness:

• Ready for merge

Additional notes

The existing redshift_pa_setup.png screenshot in Step 3 does not yet show the new field and should be refreshed.

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Preview links (active after the build_preview check completes)

Modified Files

• https://docs-staging.datadoghq.com/epetzel/docs-redshift-copy-iam-role/experiments/guide/connecting_a_data_warehouse