refactor(Django): improve API documentation

[raphodn]

What

Changes regarding the API :

• move the docs endpoint out of the v1 namespace
• add serializers to login & session endpoints
• status endpoint : add a tag, and a response serializer
• fix Proof default queryset
• add APPEND_SLASH = False to avoid redirects, and trailing_slash=False in DRF setup
• fix decimal warning

Todo

- open_prices/api/auth/views.py: Warning [SessionView]: could not resolve authenticator <class 'open_prices.common.authentication.CustomAuthentication'>. There was no OpenApiAuthenticationExtension registered for that class. Try creating one by subclassing it. Ignoring for now.
- open_prices/api/prices/views.py: Warning [PriceViewSet]: could not resolve authenticator <class 'open_prices.common.authentication.CustomAuthentication'>. There was no OpenApiAuthenticationExtension registered for that class. Try creating one by subclassing it. Ignoring for now.
- open_prices/api/proofs/views.py: Warning [ProofViewSet]: could not resolve authenticator <class 'open_prices.common.authentication.CustomAuthentication'>. There was no OpenApiAuthenticationExtension registered for that class. Try creating one by subclassing it. Ignoring for now.