Update Okular website to use Hugo

This work was mostly done by @flyingflamingo during this SoK. You can get an overview of his work in:

• https://www.pablomarcos.me/es/posts/concursos/season-of-kde-2021/sok-report-january/
• https://www.pablomarcos.me/es/posts/concursos/season-of-kde-2021/sok-report-february/
• https://www.pablomarcos.me/es/posts/concursos/season-of-kde-2021/sok-report-march/

This MR is still a draft because:

• [ ]: It's missing the translations.py script but I'm currently moving it to a new repo as a reusable python package
• [ ]: Instruction in the readme about how to build the website (basically hugo serve)

But the content can already be reviewed. CC @teams/vdg @aacid @davidhurka @sander

The result can be seen in action at https://api.carlschwan.eu/