Add role based auth to CloudWatch exporter doc

[minkimipt]

Role based authentication is added for CloudWatch exporter that allows to authenticate with users' AWS accounts and we need to document configuration that needs to be done on the user side to make it work.

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Fixes #3155

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[github-actions]

Allow 10 minutes from last push for the staging site to build. If the link doesn't work, try using incognito mode instead. For internal reviewers, check web-documentation repo actions for staging build status. Link to build for this PR: http://docs-dev.timescale.com/docs-update-cloudwatch-exporter-ingetrations

[minkimipt]

@niconosenzo why do we call it trust policy here if it's a role?

[niconosenzo]

🤔 it was the trust policy that began in line 152 of my commit. Will double check though

[minkimipt]

I think if we want to be aligned with how AWS calls them in the blog, we need to call it a role with a trust policy

3. Create a role in the shared_content account that provides ReadOnlyAccess to all objects in pics bucket. Users federated by OIDC provider are allowed to assume this role.

 "Version": "2012-10-17",
  "Statement": [
    {
      "Effect": "Allow",
      "Principal": {
        "Federated": "arn:aws:iam::shared_content_account_id:oidc-provider/oidc.eks.us-east-1.amazonaws.com/id/oidc-id"
      },
      "Action": "sts:AssumeRoleWithWebIdentity",
      "Condition": {
        "StringEquals": {
          "oidc.eks.us-east-1.amazonaws.com/id/oidc-id:aud": "sts.amazonaws.com"
        }
      }
    }
  ]
}

[niconosenzo]

TBH, for me it's always easier to split a role among permission policies and trust policies. But yeah we can call it a role if that makes more sense.

[github-actions]

Oh no, the docs build failed! To see why visit the failing build (link only accessible to Timescalers).

[minkimipt]

@billy-the-fish last docs build that I saw looked good to me. The PR was actually started by me, that's why I'm not allowed to approve it, but I think it looks good. @niconosenzo do you want to have a final look to?

[niconosenzo]

Thanks!

[billy-the-fish]

LGTM