Place plugin configuration page in the 'System Configuration' section

[MarkEWaite]

Place plugin configuration page in the 'System Configuration' section

Previously it was placed in the "Uncategorized" section of the "Manage Jenkins" page. It makes sense to have it in the same section as "Clouds" and "Nodes".

Testing done

Confirmed that before the Docker configuration page was in "Uncategorized" before this change and is in "System Configuration" after this change.

Before this pull request

After this pull request

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