[Doc] Interaction with a model and a solution

[ocots]

Add in the documentation a manual on the possible interactions with a model (that is an optimal control problem) and a solution.

The manual must include docstrings and examples.

Remark. Maybe split in two pages.

Model

• Explain the Model type and the associated getters. Do not mention methods that takes a PreModel as input.
• Retrieve a constraint
• Print
• Build a solution
• solve
• ...

Solution

• Print
• Plot
• Type
• Getters
• dual method
• Build a solution: link to the explanation in the Model manual.
• ...