Make Secret Dection required for docs projects

Why is this change being made?

Add a new policy to the AppSec inventory to make Secret Detection required for projects categorized as docs (containing or generating any kind of documentation, even runbooks). Documentation, and especially runbooks, are good candidates to leave behind some secrets in pasted snippets of code. Without Secret Detection running, we can't spot these secrets and risk to publish content with them.
A new policy would be added to GIB if this MR gets approved and merged.

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
  • 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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@laurence.bierner @jritchey @ankelly @vdesousa Could you please review? Thanks