Feat: add preview environment & update theme

[almereyda]

Supersedes #1

This PR builds the IDN website from a synchronised fork with an adapted theme in another synchronised fork.

• Add Hugo workflow for preview environment to build dev branch and push to Github pages

• Add ReadMe

• Update submodule to use idn-theme

• Update package.json and yarn.lock

• Remove netlify configuration

[almereyda]

We are not sure if the Netlify configuration was used for any kind of preview environment, as there was no documentation, why we removed it.