Implement documentation for polynomials and barycentric formula

[pasabanov]

Problem or Motivation

The barycentric formula was updated in ALFI-lib/ALFI#47.

The changes must be reflected in the documentation.

The barycentric formula is based on the improved Lagrange formula, which is based on the Lagrange formula, so the documentations for these formulas should be related.

The documentation for the Newton formula should also be implemented.

Solution or Suggestion

Tasks:

1. • Add documentation for the Lagrange formula.
2. • Add documentation for the improved Lagrange formula.
3. • Add documentation for the Newton formula.
4. • Add documentation for the barycentric formula.