Publish Python SDK docs to GitHub Pages

[zeevdr]

Description

The Python SDK is the only SDK whose API docs are not published. mkdocs.yml exists in this repo, but the GitHub Pages site returns 404, while Go (pkg.go.dev), TypeScript (opendecree.github.io/decree-typescript), and the Admin UI all have live docs. Launch messaging promises "SDK docs for Go/Python/TypeScript reachable in one click each" — Python currently breaks that promise.

Acceptance criteria

• mkdocs site builds cleanly via the Makefile/docs target
• A CI workflow builds and deploys the docs to GitHub Pages on release/main
• GitHub Pages is enabled with the correct source and the site returns 200
• The published docs URL is linked from the decree-python README
• The docs URL is consistent with how the other SDKs link their docs