Normalise code examples

[bonjourmauko]

There are some examples in the documentation that are written for the country-template while other are for openfisca-france.

Imagine the terrified face of users when they see this.

It should be improved :

• Fix variables naming section
• Update "writing YAML tests"
• Reword "The system for France is currently the only one well implemented" in key-concepts/tax_and_benefit_system (it's not super friendly)
• Don't equate two unrelated concepts in key-concepts/variables: "basic income (in France, RSA)"
• Don't mention Paris specifically, use a generic concept like "postal code" in key-concepts/variables
• Use the country template entities as examples rather than France, in key-concepts/person,_entities,_role
• Ditto for roles
• Improve the section title "Application: module used by OpenFisca" in key-concepts/person,_entities,_role
• use country template variables in key-concepts/input_data
• use a country template test case in key-concepts/input_data
• don't recommend using the "modality" of an enum in key-concepts/input_data
• use a country template example in key-concepts/simulation

[MattiSG]

It seems quite obvious to me that examples should be from the Country template.