expose dlt commands to notebooks as magics + improve colab experience

[rudolfix]

Background
See how sql-mesh is doing that. amazing: https://sqlmesh.readthedocs.io/en/stable/reference/notebook/
if we do that we allow our users to easily add verified sources in notebooks and inspect working pipelines. we may also think about some notebook-specific magic ie. showing current config and secrets

Tasks

1. • add magics for init, pipeline and other commands
2. • there's a known limitation on Colab - their file editor cannot show files starting with . the result is that users do not see the .dlt folder with secrets and configs. we should solve this somehow. ie. by dlt init that detects running on colab (there's a helper) and then creates a link ie. _dlt that is visible
3. • add a demo colab notebook and documentation to reference section of the docs