nixos/datadog: Don't recommend dd_url for sites, add proper option
nixos/datadog: Don't recommend dd_url for sites, add proper option

Turns out, `dd_url` should only be used in proxy scenarios, not to point
datadog to their EU endpoint - `site` should be used for that.

The `dd_url` setting doesn't affect APM, Logs or Live Process intake
which have their own "*_dd_url" settings.

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