Small reorganization, formatting updates for Data Guide to SM estimation

Why is this change being made?

• To help the reader, there are a couple of changes to reorganize the content and headings.
  • Move the knowns and unknowns into the overall methodologies section
  • Make the current methodology and visualization sub-headings below the intro
• To align with the handbook style guide, there are also a few formatting changes
  • Use sentence case for headings
  • Remove contraction
  • Spell out number
  • Fix typo
  • Observe ~100 character line limit

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
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added, and the content is SAFE
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • 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
  • 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
  • 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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