Add explanation of confidence intervals/quantiles to methodology description

[FrankKr]

Predictions made with openSTEF also have a confidence interval, either in terms of stdev, or using quantile regression. Add an explanation of these concepts to the documentation of OpenSTEF.

See also #27

There are two goals:

• Short explanation how quantile forecasts are produced by OpenSTEF.
• Clear explanation how to read quantile forecast plots and what conclusion can and cannot be drawn from them.

[FrankKr]

First version placed on internal confluence, retrieving internal reviews before adding it to AIFES.

Internal link: https://alliander.atlassian.net/wiki/spaces/SASOG/pages/3648749855/OpenSTEF+Uncertainty+Estimation

[FrankKr]

https://github.com/OpenSTEF/openstef/pull/465