[JOSS] Add additional usage examples in the docs

[damar-wicaksono]

Following a feedback in the JOSS submission review additional examples should be added to the repository/docs. I would put these examples in the Getting Started section.

[damar-wicaksono]

This issue has been resolved by PR #297.

Getting started section of the docs has been reorganized; it now includes four tutorials (two were already there before: create built-in test functions and create custom functions). The new tutorials are about how to use functions in UQTestFuns in two different contexts: sensitivity analysis and reliability analysis.