Allow to preview PRs

[patrick91]

We had this kinda working for the previous setup. We can improve this now since we have SSR, we can allow PRs with a specific label (ie. preview-docs) or by core devs to be previewed using a link like this:

https://strawberry.rocks/docs/pr/123/general/schema-basics

where 123 is the id of the PR. We could use a subdomain too, but I think it is easier to use one :)