doc: clarify that the cloud endpoint must use/v1

[thiswillbeyourgithub]

I didn't see it documented anywhere?

Signed-off-by: thiswillbeyourgithub [email protected]

[vercel[bot]]

@thiswillbeyourgithub is attempting to deploy a commit to the umami-software Team on Vercel.

A member of the Team first needs to authorize it.

[ceviixx]

/authentification is only on self hosted available. So there is /v1/ not required.