Onboarding data handling

[CPatchane]

Add documentation about user data management by the onboarding (.md file). This documentation describe the existing working and data flow. A proposition is given to:

• Detect the step according to the user document context
• Describe views behaviours with data
• Give a way to update onboarding steps in the future

Important points:

• welcome, agreement and password steps are processing before the login, so the only one user data used is the public_name.
  • Endpoint to use: POST /register
• Other steps will require user authentication to request other user informations from the user document (for pre filled features).
  • Endpoint to use: PUT /register

[gregorylegarec]

Thx @CPatchane !