Updates incident guidance with new modal and clarifies paging

Why is this change being made?

This change is in response to a comment that was made by @johnhope that our handbook page is a bit dated for the incident declare dialog

gitlab-com/gl-infra/production#8457 (comment 1294721466)

• Guides the user to leave the "page boxes" checked, unless they know what they are doing
• Removes the text Incident Managers should **NOT** be paged for severity {3,4} since we never page for sev3 or sev4 now
• Recommends severity 3 for most internally facing issues like deployment blockers.

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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