Adds guide to explain r53 resolver dns firewall configuration applied to MP VPCs

[richgreen-moj]

A reference to the issue / Description of it

#9037

How does this PR fix the problem?

This PR adds a guide to explain the R53 resolver dns firewall configuration applied to MP VPCs

How has this been tested?

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

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Deployment Plan / Instructions

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

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Checklist (check x in [ ] of list items)

• I have performed a self-review of my own code
• 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)

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[github-actions]

Trivy Scan Success

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Trivy will check the following folders:

</details> #### `Checkov Scan` Success
<details><summary>Show Output</summary>

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CTFLint 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)
tflint will check the following folders:

Trivy Scan Success

Show Output

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Trivy will check the following folders:

[markgov]

LGTM
but did ponder over the then we will over we will then in the alerting section