docs: release process, PR template, and CI improvements

[paololucchino]

Description

• README: fix broken publish-s3.yml badge (workflow was deleted); add a Release process section and commit prefix table explaining how fix:, feat:, etc. map to version bumps
• PR template: add checklist item for Conventional Commits
• python-package.yml: run CI on all PRs targeting main (not just pushes)
• tests/conftest.py: replace hardcoded data_bridges_api_config.yaml with config_from_env() so fixtures work in CI using repo secrets

Type of Change

• 📚 Examples / docs / tutorials / dependencies update
• 🔧 Bug fix (non-breaking change which fixes an issue)
• 🥂 Improvement (non-breaking change which improves an existing feature)
• 🚀 New feature (non-breaking change which adds functionality)
• 💥 Breaking change (fix or feature that would cause existing functionality to change)
• 🔐 Security fix