Docstrings and changed prompt template param

[ea-rus]

Added docsting for methods exposed in API

• datasourses
• minds

Changed the way to pass prompt template for create or update mind
Used to be:

mind = client.minds.create(
    'my_mind',
    parameters={
          'prompt_template': 'do the best'
    }
)

Will be:

mind = client.minds.create(
    'my_mind',
    prompt_template='do the best'
)

[ea-rus]

@martyna-mindsdb, about changing way to pass prompt_template to mind:
Do we have docs which have to be updated?