[DOC] API reference for tags

[Mushfiqur719]

Reference Issues/PRs

Fixes #768
Depends on #767 (class-based tag system, already merged)

What does this implement/fix? Explain your changes.

This PR adds a comprehensive tag documentation page to the skpro API reference, following the same pattern established by sktime (see sktime's tags.rst).

Tags in skpro describe properties, capabilities, and behavior of objects such as estimators, distributions, and metrics. Previously, these tags were only discoverable in the source code skpro.registry._tags. This change provides a centralized documentation page listing and organizing them.

Changes:

1. New file: [docs/source/api_reference/tags.rst]

   • Introductory section explaining what tags are and how to inspect them
   • autosummary directive for the all_tags utility function
   • Organizing tags into six sections:
   • General / packaging tags
   • Probabilistic regressor tags
   • Distribution tags
   • Metric tags
   • Meta-object tags
   • Testing / CI tags
   • Each section links to the corresponding tag definitions in skpro.registry._tags

2. Modified: [docs/source/api_reference.rst]

   • Added api_reference/tags entry to the main API reference toctree (placed between metrics and base)

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

No.

Any other comments?

This follows the exact same documentation pattern used in sktime, making it easy for contributors familiar with the sktime ecosystem to navigate skpro's tag system. All 30 tags currently defined in skpro.registry._tags are covered.

[arnavk23]

@fkiraly @marrov This issue is already being worked on in #799

[fkiraly]

Hm, looks like it has been already solved in #877. Will merge both for credit.