Hosting docs and syntax on catala-lang.org

[denismerigoux]

In case somebody is motivated to change the URL of the auto-generated gitlab page https://catalalang.github.io/catala/syntax.pdf to something like https://assets.catala-lang.org/...

Here's the config :

https://docs.github.com/en/pages/configuring-a-custom-domain-for-your-github-pages-site/managing-a-custom-domain-for-your-github-pages-site

I can set up the DNS accordingly.