Documentation for the regression feature

A merge request on the ordinary polynomial regression has been created (!53 (merged)); all polynomial regression capabilities in minterpy are included in extras. While the merge request itself is still under review, we can already start the documentation process of this particular feature. The documentation of minterpy should be updated accordingly. Below are non-exhaustive list of ToDos:

• Add a new chapter on polynomial regression in the Fundamentals (resolved via MR !158 (merged))
• Update the polynomial regression in the Getting Started Guide (see Issue #176 (closed); resolved via MR !153 (merged))
• Update the API reference by extracting relevant docstrings from the ordinary regression module (see Issue #108 (closed); fixed by MR !71 (merged))
• A couple of How-to guide ideas related to the ordinary regression feature: regression using bases other than the default Lagrange basis, using different least square solver, and weighted regression.