Upgrade the Hugo version (#2134)

## Changes

- Pre-emptively fixes an issue Hugo v0.152+ would have with our
`data/docs.yml` by converting it to JSON
- Upgrades to newest Hugo (v0.155.3)

## Context

It's always good to stay up to date with dependencies. Our principal
dependency, Hugo, occasionally introduces breaking changes and it is
good to stay on top of those changes by adapting your site accordingly.

Author: dscho