Add swagger docs

[KipSigei]

To be added to the settings file:

SPECTACULAR_SETTINGS = {
    'TITLE': 'Onadata API',
    'DESCRIPTION': 'Your API Description',
    'VERSION': '1.0.0',
    'SERVE_INCLUDE_SCHEMA': False,
    'SWAGGER_UI_SETTINGS': {
        'deepLinking': True,
        'defaultModelRendering': 'model',
    }
}