Fix (generalize) API links in docs

[chaoran-chen]

In some of our docs pages, we making assumptions of where the APIs can be reached which are not necessarily true. For example, here, we write that the backend is at https://backend.<URL of the Loculus website> but that's not always the base. In our previews, it is at https://backend-<URL of the Loculus website> and in GenSpectrum, it is at https://api.loculus.genspectrum.org/backend/. We should phrase it more generally.