Refresh BIA Handbook Page

Why is this change being made?

As part of our FY24Q1 OKR we are refreshing Security Risk Handbook and Runbook documentation. This MR makes the following changes:

1. Combined 2 steps for Ad-hoc BIA procedure for brevity
2. Changed Markdown numbered lists to all 1s for dynamic updating.
3. Moved footnote to paragraph form because the footnote was rendering at the bottom of the page instead of directly under content.
4. Removed the Quality Check for MTD and Impact of System outage as they are not expected to be always in alignment
5. Removed Operating System check work as the approved_OS guidance appears to no longer be in the handbook (except for endpoint devices)
6. Added bottom-up language to demonstrate GitLabs complementary top-down and bottoms-up approach to risk management
7. Removing Tier 4 systems from periodic BIA procedures due to the low risk they present to GitLab which closes https://gitlab.com/gitlab-com/gl-security/security-assurance/security-risk-team/storm/-/issues/299+

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 DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
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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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