Update specification of dual to include constraints with meta, and update documentation for users

[rodrigomha]

Is your feature request related to a problem? Please describe.
Currently there is no clear tutorial on how to consider duals. In addition, there is no clear way of getting duals of ConstraintType that is using a meta in the model construction.

Describe the solution you'd like
Being able to specify constraints with meta extension, while clarifying how to do it in the documentation

Describe alternatives you've considered

Additional context
Still not sure how to even modify the container or template to select the right constraint.