Update to cover Non-Disclosure agreements

Why is this change being made?

Now that an NDA process exists, we want to call it out in the handbook without going into detail about the process (which is confidential).

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][transparency]
• Assign this change to the correct DRI
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