updates to incident management documentation - added summary

[SimonPPledger]

A reference to the issue / Description of it

Added summary and some clarifications to incident management runbook

How does this PR fix the problem?

Makes the documentation less confusing

How has this been tested?

Please describe the tests that you ran and provide instructions to reproduce.

have run and viewed locally

Deployment Plan / Instructions

Will this deployment impact the platform and / or services on it?

n/a

Checklist (check x in [ ] of list items)

• [x ] I have performed a self-review of my own code
• [ x] All checks have passed
• I have made corresponding changes to the documentation
• Plan and discussed how it should be deployed to PROD (If needed)

Additional comments (if any)

{Please write here}

[github-actions]

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</details> #### `Checkov Scan` Success
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Setting default tflint config...
Running tflint --init...
Installing "terraform" plugin...
Installed "terraform" (source: github.com/terraform-linters/tflint-ruleset-terraform, version: 0.9.1)
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[ewastempel]

It makes sense to me, although the key steps do not always match the heading names, but it makes sense to me (and the key steps are probably more meaningful than the current headings)