[DOC] Add formula docstring injection hooks for distribution methods

[Tusharjamdade]

Reference Issues/PRs

Fixes #689

What does this implement/fix? Explain your changes.

Adds a mechanism to inject distribution-specific closed-form formulas into method docstrings (pdf, log_pdf, cdf, pmf) via class-level hooks, handled automatically in BaseDistribution.__init_subclass__.

Does your contribution introduce a new dependency? If yes, which one?

What should a reviewer concentrate their feedback on?

• Docstring injection logic
• Method cloning safety
• Inheritance behavior

Did you add any tests for the change?

Any other comments?

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For new estimators

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[fkiraly]

Nice!

Could you kindly:

• implement this for a few distributions so we see it works
• also ensure that the docstring renders properly where no formulae are added, i.e., the value of the hook is None
• add this to all public methods, e.g., energy, log_pmf, etc.

[Tusharjamdade]

Hi @fkiraly, please take a look.

[fkiraly]

why do you include the header "closed form"?

[fkiraly]

this didn't work. Please check the doc build yourself before you ask for review. You can look at it by clicking on the doc build PR, or by building locally. This is how the result looks like:

Also, code formatting is failing. See https://www.sktime.net/en/stable/developer_guide/coding_standards.html for how to set it up locally.