Introduce process for uploading Demos

Why is this change being made?

In order to encourage transparency and border visibility of the work done by the teams, this MR adds a section on Demos to the Fulfillment Handbook. The advantages are listed and an example is given. See this conversation (internal link) for background info.

Author Checklist

• Provided a concise title for the 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 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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