docs: add new vignettes

[MMenchero]

Description

The current documentation is missing some tutorials that the Python SDK has.

New vignettes:

• Setting up your API key
• Data requirements
• Prediction Intervals
• Quantiles This was added in the quantiles PR
• Long-Horizon Forecasting
• Special Topics (handling missing values and specifying the frequency of the data)

The vignettes aim to replicate the Python documentation, However, to minimize the number of mock files, they might use a different dataset. Priority will be given to the electricity dataset included in nixtlar.