[DOC] Complete Related Software documentation page

[maniktyagi04]

Problem

docs/source/related_software.rst is linked from the README.md and the documentation navigation as "Related Software", but the file contains only a bare TODO placeholder. The resulting published page at https://skpro.readthedocs.io/en/latest/related_software.html is empty.

Current State

.. _related_software:

================
Related Software
================

TODO

Expected State

The page should list related software packages in the probabilistic prediction and scientific Python ecosystem, with brief descriptions of their relationship to skpro.

Suggested Content

• sktime – Unified framework for time series ML; maintained by the same community, integrates with skpro for probabilistic forecasting.
• scikit-learn – skpro is scikit-learn-compatible and extends it with probabilistic prediction capabilities.
• MAPIE – Uncertainty quantification via conformal prediction; interfaced in skpro.
• ngboost – Natural Gradient Boosting for probabilistic predictions; interfaced in skpro.
• cyclic-boosting – Probabilistic boosting algorithms; interfaced in skpro.
• lifelines – Survival analysis library for Python.
• pymc – Probabilistic programming in Python.