documentation: Update datadog sink's configuration link

[rpriyanshu9]

Summary

Update URL link to configuration for the datadog sink.

Motivation

While searching for datadog sink's configuration options, this link redirects me to the example.yaml file which does not contain the datadog keys as mentioned in the datadog sink's README.md. So, I've updated the link to an older version of example.yaml and also explained to append datadog_ to the struct tags present in the code to get the keys.

Test plan

This does not include a code change, so no tests written.

Rollout/monitoring/revert plan

Just reverting the PR would do.

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