Add provider-specific setup documentation

## The problem

We need to include a page per provider in the documentation with some REF-specific information/snippets.

For example, handling exisiting ESMValTool deployments or overriding config https://github.com/Climate-REF/climate-ref/issues/289#issuecomment-3749672679

## Definition of "done"

A page per producer in docs

## Additional context