Update circle.yml to version 2

[Ryuno-Ki]

See documentation

It allows finer control on the build step.
Maybe we can add a badge on the README.md while we are at it.

[Ryuno-Ki]

So for other @Sabayon/sabayon-web-developers / @Sabayon/scr-devs to ease the migration.

There's a migration guide but I find it so lala.
What's more exciting is the API endpoint for a translation of the config!

Short: Log in into CircleCI with GitHub and visit https://circleci.com/api/v1.1/project/github/Sabayon/sabayon-website-next/config-translation to download an auto-migrated config.yml (replace the repo with yours).

This is not ready for production though (because the deployment is missing).

For that, I looked more into sample configs to learn the structure.

I wasn't exactly sure, what to do with the pre-built CirlceCI docker images, especially since there seem to be no official Hugo docker image, so I went with one from its community.

For learning the workflows part, it helped to sneak into demo workflows.

I hope that helps us migrating the existing configs quickly.

Shout out to @circleci/circleci-docs for their documentation!