Add scaffolding for `handbook/it` and `handbook/business-technology/it` pages

Why is this change being made?

This adds the handbook page scaffolding for the Information Technology (IT) department to create new information architecture for topics that our team members are looking to get help with. This also adds a new handbook vision/mission page for our newly renamed department and unified IT teams.

Highlights:

•  This unblocks https://gitlab.com/gitlab-com/business-technology/engineering/operations/issue-tracker/-/issues/41 to add more info to our handbook.
•  This improves the information architecture and discoverability of content for end users (on one page) and for our inter-department how we work (on a separate page). This is similar to the https://about.gitlab.com/handbook/security and https://about.gitlab.com/handbook/engineering/security/ handbook information architecture.
• It’s okay if we have TODO’s committed for now. We can rename stuff later, so don’t get stuck on section titles, etc.
• This MR won’t remove any existing handbook pages, only cross-link to them for progressive iteration later.

Related https://gitlab.com/gitlab-com/Finance-Division/finance/-/issues/4985

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
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this change to the correct DRI(s)
  • 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 in on the page being edited.
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies.
• 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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