[DOCS] Benchmarks

[carlomazzaferro]

Document benchmarks of this library agains:

1. Barebones forecasting methodologies without reconciliation
2. Hyndamns R's implementation (https://cran.r-project.org/web/packages/hts/index.html)
3. Methods against each other on a variety of datasets

Ideally, benchmarks would be easily reproducible and version controlled. In a top level directory named benchmarks we implement the benchmarks described. In a documentation page, we describe results and describe how new benchmarks can be added