Publish Python SDK docs site via mkdocs-material on GH Pages

[zeevdr]

Description

Python SDK convention: API reference + usage guide on Read-the-Docs or mkdocs-material on GH Pages. Currently README-only. mkdocs-material is the near-universal Python SDK choice — low-maintenance.

Acceptance criteria

• mkdocs.yml + docs/ structure
• mkdocs-material theme with org-branded colors
• API reference auto-generated from docstrings via mkdocstrings
• Usage guide: install, connect, basic CRUD, schema definition
• Deploy workflow on tag + main → GH Pages
• Link from README (opendecree.github.io/decree-python)