Bring back missing docs

[olayway]

From @rufuspollock:

https://github.com/datahubio/docs is private and unused. Initially was source for datahub.io/docs but that is now ...

https://github.com/datahubio/datahub-cloud-template-dataset/tree/main/docs

I note that https://github.com/datahubio/docs is a small amount of datahub v3 docs plus mostly old v2 docs which were updated in v2.5 / v3a (from 2023)

We have a copy or original v2 docs here https://github.com/datahubio/datahub-v2-docs

And somewhere in datahub-next repo we have the updated version of this from pre v3 release.