Add contributing file to all repositories

All repositories under the Keycloak organization should have a common contributing file. This should be based on https://github.com/keycloak/keycloak/blob/main/CONTRIBUTING.md, but should be make generic for all repositories.

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

Read the existing Keycloak CONTRIBUTING.md linked in the issue and compare it with the .github repository context. Create a generic default CONTRIBUTING.md in the .github repository, removing repository-specific guidance while preserving useful common contribution instructions. Done means the default file is present and applies to repositories under the Keycloak organization.

Written by the indexing model from the issue text.