Add readthedocs.io documentation

[leo-schick]

A readthedocs.io. documentation is needed with samples how to use the package.

[leo-schick]

When mara/mara-pipelines#69 is merged and published, a sample should be added in README and docs how to run dbt on a separate container through a Mara pipeline. For example:

from mara_pipelines.commands.sql import ExecuteSQL
from mara_pipelines.pipelines import Pipeline, Task
from mara_dbt.commands import DbtRun


pipeline = Pipeline(
    id='dwh_dbt',
    description="Runs additional dbt transformations on the data warehouse")

pipeline.add_initial(
    Task(id='initialize_schemas',
         description="Create the schema",
         max_retries=3,
         commands=[ExecuteSQL(sql_statement="""
EXEC util.drop_schema_force 'dbt_next'
GO
CREATE SCHEMA dbt_next
GO
""")]))

pipeline.add(
    Task(id='run',
         description='Run dbt run',
         commands=[DbtRun()],
         context='dbt-project-dwh'))

pipeline.add_final(
    Task(id='replace_schema',
         description="",
         commands=[ExecuteSQL("EXEC util.replace_schema 'dbt_next', 'dbt'")]))