Update Hugo and docsy theme

The site uses the [docsy theme](https://github.com/google/docsy) as a submodule and [hugo](https://gohugo.io/) for building the site. The [specified commit for docsy](https://github.com/google/docsy/tree/2bedb2ff278eddba4a7fc4441ca6bf7a42caeb68) is from 2020. The version of hugo currently used in the deployment script is `0.74.2`, which is also from 2020. Newer versions of each are available, but contain some breaking changes that affect the look and feel of the site. Update to the latest versions of each, making adjustments as necessary to keep the site look and feel as similar as possible.