Theme and organizational improvements for the markdown guides

[david-waltermire]

Committer Notes

• Updated to newer version of the hugo-uswds theme and made configuration changes based on theme changes.
• Reworked side navigation to provide a better integration of general vs model-specific guide content.
• Setup hugo to use the uswds theme as a module.

All Submissions:

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• [ ] Have you checked to ensure there aren't other open Pull Requests for the same update/change?
• [ ] Have you squashed any non-relevant commits and commit messages? [instructions]
• [ ] Have you added an explanation of what your changes do and why you'd like us to include them?
• [ ] If applicable, have all FedRAMP Documents Related to OSCAL Adoption affected by the changes in this issue have been updated.?
• [ ] If applicable, does this PR reference the issue it addresses and explain how it addresses the issue?

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