[DOC] add explicit formulae to docstrings of distributions

We should add, where known, explicit formulae to the docstrings of distribution methods. This is done by populating the `_formula_docs` attribute of the given distribution, see the extension template in `extension_templates.distributions` for a detail explanation, and `Normal` for an example of a populated distribution. Contributors can help by: * adding formulae to a distribution which does not have them yet * adding formulae for more methods in a distribution which already has them Please post here before starting, to avoid duplication: which distribution and method(s) you are working on. If opening a PR, please provide a reference for the formula - either a literature reference, or your own compitation. No AI generated formulae please, since these tend to be hallucinated. Recipe for finding distributions that have not yet been dealt with: ```python from skpro.registry import all_objects all_dists = all_objects("distribution", return_names=False) dists_without_formula = [d for d in all_dists if not hasattr(d, "_formula_docs") or len(d._formula_docs) <6] ```