Add Contributing.md

The Contributing.md-file was added. Content are the contribution-guidelines as discussed.

Ticket: planning#211 (closed)

Rendered preview:

• https://gitlab.opencode.de/fitko/matrix-g2x/solution-architecture/-/blob/hcunz/contributing/CONTRIBUTING.md
• https://gitlab.opencode.de/fitko/matrix-g2x/solution-architecture/-/blob/hcunz/contributing/docs/architecture.md

Related: matrix-g2c-pilot-project-template!17