chore(doc): add documentation of release process

[rubenhoenle]

Description

relates to STACKITTPR-200

Checklist

• Issue was linked above
• No generated code was adjusted manually (check comments in file header)
• Changelogs and versioning
  • Changelog in root directory was adjusted (see here)
  • Changelog of the service(s) was adjusted (see e.g. here)
  • pyproject.toml of the service(s) was adjusted (see e.g. here)
• Examples were added / adjusted (see examples/ directory)
• Unit tests got implemented or updated
• Unit tests are passing: make test (will be checked by CI)
• No linter issues: make lint (will be checked by CI)