docs: update contributing guidelines

Issue: planning#211 (closed)

Rendered Preview:

• https://gitlab.opencode.de/fitko/matrix-g2x/matrix-g2c-pilot-project-template/-/blob/kim/docs/211-update-contributing/CONTRIBUTING.md
• https://gitlab.opencode.de/fitko/matrix-g2x/matrix-g2c-pilot-project-template/-/blob/kim/docs/211-update-contributing/docs/workflow.md

This MR updates the contributing guidelines in conjunction with solution-architecture!4