Update plugin configuration docs

[nikita-tkachenko-datadog]

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What does this PR do?

Updates plugin documentation following the configuration structure update.
Instructions for configuring the plugin via UI, Groovy scripts and environment variables are updated.

The configuration changes are backward-compatible, so existing scripts and environment variables continue to work, but going forward the users will benefit from utilising the new structure because it is simpler and clearer.

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• Feature or bug fix MUST have appropriate tests (unit, integration, etc...)
• PR title must be written as a CHANGELOG entry (see why)
• Files changes must correspond to the primary purpose of the PR as described in the title (small unrelated changes should have their own PR)
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