Update details on tax witholding rates in the Netherlands

Why is this change being made?

The current detail of tax withholding rates in the handbook is outdated. This MR updates this information, along with some more detail around how the tax withholding rates are being calculated.

This question came up in Slack, and the MR adds background information on the tax rates.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
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• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
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