Update France's Sick Time reporting policies and procedures

Why is this change being made?

Team members in France are required to provide "proof" of their need for sick leave, even if it is just for one day. These instructions were not in the handbook, so I've added them to help provide this guidance for team members.

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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