Improve contributing guide with Sonar configuration

The contributing.md file could be improved to include Sonar configuration as described by @sdinot in the following description. However, the Sonar configuration could be highlighted as an optional step of a contribution since the changes performed in !177 (merged).

After updating the contributing guide, the Please, configure SonarQube! message in .gitlab-ci.yml (line 50) could be replaced by Please, configure SonarQube by following steps described in the contribution guide: https://www.orekit.org/site-orekit-latest/contributing.html