Update documentation with supported `hugo` version

[aj-stein-gsa]

Per discussion today with staff trying to set up this repo for local development and testing of branch and PR versions of the site for review and approval, team members identified the README information is not clear on required and recommended dependency versions.

• Fix the README to update the minimally required viable hugo version for testing and deployment
• Point developers to the .hugo-version file in the repo to align with the exact version used by GitHub Actions CI/CD.