Incorporate Hugo Docsy Theme

[ajschmidt8]

As part of rapidsai/docs#388, we want to move all of the docs.rapids.ai content into the new rapids.ai Hugo site.

To avoid having to create our own Hugo templates for the non-library documentation that's currently on docs.rapids.ai (like these pages), we can simply adopt the Docsy theme.

The Docsy theme has support for landing pages, documentation, blogs, etc., so it will be more than suitable for our use case.

We can customize the Docsy theme (e.g. colors and other styles) to ensure consistent branding across the page types.