Create notebook about forecasting strategies

[alex-hse-repository]

🚀 Feature Request

Create notebook about forecasting strategies in etna

Proposal

• Create notebook about the forecasting strategies in etna, it should contain examples with:

1. Pipeline
2. AutoregressivePipeline
3. DirectEnsemble
4. assemble_pipelines + DirectEnsemble

• Notebook should clearly explain the difference between the strategies and teach how to use them in our library

Test cases

No response

Additional context

No response