Dedicated docs section for Deployment-dependent parametrisation using ENVs (VAR)

[aklikic]

Currently this is documented under S2S and Subscribe to Topic but is applicable also for Publishing to a Topic.
It is very hard to find so maybe a dedicated section directly under Publish-Subscribe would make more sense to consilidate the information.
We could maybe also change the name of the section to: Deployment-dependent configuration - externalising values