Add ATX Header Support for `terraform-docs`

[michaelsaki]

🗣 Description

This pull request:

• Clones @mcdonnnj's forked branch from terraform-docs.
• Builds and installs the binary from the clone.

💭 Motivation and context

This temporarily resolves #196 until @mcdonnnj's changes get merged into terraform-docs, at which point we can change back over to the standard of installing terraform-docs.

🧪 Testing

All automated tests passed. I also used act for local testing to make sure the workflow would behave like it should.

✅ Pre-approval checklist

• [x] This PR has an informative and human-readable title.
• [x] Changes are limited to a single goal - eschew scope creep!
• [x] If applicable, All future TODOs are captured in issues, which are referenced in the PR description.
• [x] All relevant type-of-change labels have been added.
• [x] I have read the CONTRIBUTING document.
• [x] These code changes follow cisagov code standards.
• [x] All relevant repo and/or project documentation has been updated to reflect the changes in this PR.
• [x] All new and existing tests pass.

✅ Pre-merge checklist

• [ ] Revert dependencies to default branches.
• [ ] Finalize version.

✅ Post-merge checklist

• [ ] Create a release.

[michaelsaki]

Thanks for taking care of this! 👍

Can you please confirm that you re-tested this workflow after making the changes requested during the PR review?

You are welcome! Yes, I ran any changes to the workflow in act, everything checked out locally.