[DOC] Complete Related Software documentation page

[maniktyagi04]

Reference Issues/PRs

Fixes #998.

What does this implement/fix? Explain your changes.

This PR populates the previously empty docs/source/related_software.rst file with a curated list of software packages related to skpro in the probabilistic prediction and scientific Python ecosystem.

The updated page now includes:

• sktime (unified framework integration)
• scikit-learn (compatibility and extension)
• MAPIE (uncertainty quantification)
• ngboost and cyclic-boosting (native interfaces)
• lifelines and pymc (broader ecosystem)

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

No.

What should a reviewer concentrate their feedback on?

• RST formatting and link correctness.
• Completeness of the suggested package list.

Did you add any tests for the change?

No, this is a documentation-only change.

PR checklist

For all contributions

• I've added myself to the list of contributors with any new badges I've earned :-)
• The PR title starts with either [ENH], [MNT], [DOC], or [BUG].