[DOCS] Getting started

[felipeffm]

I think a smother introduction to the library with examples can help new user get used with prophet verse interface.

Suggestion: Create a getting started guide briefly explaining how to apply the methods, the bigger picture of the main parameters, andreference the notebooks with examples.

├── 1-univariate_prediction.md ├── 1.1-change_priors.md ├── 1.2-exogenous_variables copy.md ├── 1.3-non_linear_exogenous_variables.md

├── 2-multivariate_prediction.md ├── 2.1-change_priors.md ├── 2.2-exogenous_variables.md ├── 2.3-non_linear_exogenous_variables copy.md ├── 2.4-shared_coefficients.md

└── 5-hierarchical-forecasting.md

├── 4-optimization.md