[PRP-349] Add inline comments to terraform child modules

[rocketnova]

Ticket

• https://wicmtdp.atlassian.net/browse/PRP-349
• https://wicmtdp.atlassian.net/browse/PRP-272

Changes

What was added, updated, or removed in this PR.

• Add inline comments to terraform child modules to make the codebase easier to approach without any context
• Remove hard-coding of SSL arn in service module
• Refactor DNS module to be more abstracted
• Make outputs consistently strings instead of complex objects when possible
• Refactor variables and outputs for consistency
• Remove unused database access IAM policy
• Move cognito-staff-user module out of child modules

Context for reviewers

Testing instructions, background context, more in-depth details of the implementation, and anything else you'd like to call out or ask reviewers. Explain how the changes were verified.

This PR audits all the child modules in /infra/modules. It attempts to make them more consistent and approachable for someone new to the codebase by:

• Adding high level doc comments to the top of each main.tf file
• Grouping long modules into sections and adding inline comments for each section
• Making variables and outputs more consistent across modules

Exception

I didn't modify the infra/modules/iam child module because I would like to propose removing it altogether in PRP-350 and letting anyone who uses this project manage IAM permissions themselves. I should have organized that module by resource before and we don't have time at the moment to refactor.

Notes

As this module is built off of #107, I would like to review this now, but merge it after the pilot ends.

Testing

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