chore(release): add PR-based release workflows

[mandaiy]

Summary

Add PR-based release automation workflows and document the process. Also update pyproject.toml version to 0.5.1.

Changes

• add .github/workflows/release-prepare.yml to create release-preparation PRs from a manual version input
• add .github/workflows/release-publish.yml to publish a release from project.version when pyproject.toml changes on main
• add docs/release_process.md and update docs/repository_structure.md
• bump pyproject.toml version to 0.5.1

Testing

• make format
• make lint
• make test

Checklist

• I updated relevant documentation under docs/.
• PR title follows Conventional Commits (e.g., feat: ..., fix: ...).
• I verified that the changes are backward compatible, or documented breaking changes.
• I confirmed this PR is ready for review.