Integrate auto-generated documentation for Gazebo Python bindings into the main docs site

## Summary

Add auto-generated Python API documentation for Gazebo libraries with Python bindings, integrated into the main documentation site.

## Motivation

Gazebo libraries increasingly provide Python bindings (sdformat, gz-math, gz-msgs, gz-transport), but there's no centralized API documentation for Python users. This makes it difficult for Python developers to discover and use the available APIs.

## Context

It could follow the existing Doxygen (C++ API) documentation pattern:

1. **Nightly CI workflow** to build Python documentation using Sphinx autodoc
2. **Automatic deployment** to `gazebosim.org/api/python/{library}/{version}/`
3. **Integration** with main docs via Library Reference page links
4. **Prototype** with sdformat, then expand to other libraries

## Technical Approach suggestions

- Use Sphinx with autodoc extension to generate documentation from installed Python modules
- Follow the existing `nightly-upload.yml` workflow pattern for C++ Doxygen docs
- Deploy to `/api-python/` nested under the existing `/api/` structure
- Build for each active distribution (Fortress, Harmonic, Ionic, Jetty)