Clarify Germany's Sick Leave Reporting Guidelines

Why is this change being made?

Per this Slack message, the handbook's guidance for team members was unclear, leading to confusion regarding when an AU certificate was required.

I've updated the guidance to better explain reporting guidelines for team members experiencing illnesses less than 3 days and those experiencing illnesses more than three days, and when the AU certificate will be required.

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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  • The when to get approval handbook section explains the workflow in more detail
• 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
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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