Enhance DD_API_KEY_SECRET_ARN description with JSON support

[litianningdatadog]

What does this PR do? What is the motivation?

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Additional notes

[chatgpt-codex-connector]

💡 Codex Review

documentation/content/en/serverless/guide/agent_configuration.md

Line 15 in 294a32f

| `DD_API_KEY_SECRET_ARN` | The environment variable with your Datadog API key, using an AWS Secrets Manager secret. **One** key environment variable is required. Starting v96, we can now extract the API key from a JSON-formatted Secrets Manager secret. If the secret value is a JSON object containing the field `dd_api_key` (hardcoded, not configurable), its value is used as the API key; otherwise the raw secret string is used as before. See [serverless CLI environment variables][7]. |

Update setup pages that still reject JSON secrets

When this new v96 behavior is published here, users following the primary Lambda setup pages still get the opposite instruction: a repo-wide search shows content/en/serverless/aws_lambda/instrumentation/nodejs.md:221, python.md:211, and many other serverless/application-security pages still say the DD_API_KEY_SECRET_ARN secret must be plaintext “not a JSON blob.” Since those pages are the install paths where users create the secret, leaving them unchanged makes the docs contradictory and will continue steering v96 users away from the newly documented dd_api_key JSON format.

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[duncanista]

approving from serverless, lets wait on docs to approve

[estherk15]

Left one suggestion on wording. If it looks ok, I can commit and merge for you. For future docs PRs, can you add a / to your branch name?