Add code of conduct file to all repositories

All repositories under the Keycloak organization should have a common code of conduct. We need to research and review what is the best option for us.

This should be done by adding a default CODE_OF_CONDUCT.md file to the https://github.com/keycloak/.github repository.

Start in the keycloak/.github repository and review the available code-of-conduct options with the project stakeholders. Add the agreed common content as CODE_OF_CONDUCT.md; the work is done when the default file is present there for the organization’s repositories.

Written by the indexing model from the issue text.