Revise docs for V2 compliance

[edoaltamura]

What should we add?

Revise the documentation to reflect the changes to adopting V2 primitives by default.

[edoaltamura]

In the revised docs one could add a subsection on using non-primitive modes for workloads, e.g. Session with VQC like described in issue #770.

This update could also cover related issues, such as those in #773: reshaping SamplerQNN output via interpret Callable.

Finally, more details on loss functions with references and equations, suggested in #821.

[edoaltamura]

Version 0.8.1 contains these fixes, so this issue may be closed.