Remove Secure Frontend Chapter from Docs

Why is this change being made?

Removing Secure Frontend Chapter from since this format never really picked up speed and is no longer something we want to pursue with the ongoing restructuring of the Teams in Secure.

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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