Prepare active repos for contributions

We want to enable the community to contribute to the Observatorium project. In order to do so, all active repositories should contain some minimal information.

AC:

• create CONTRIBUTING.md and CODE_OF_CONDUCT.md files in all active repositories in the Observatorium GitHub org: observatorium/thanos-receive-controller, observatorium/observatorium (to be renamed observatorium/api), observatorium/deployments (to be renamed observatorium/observatorium), observatorium/thanos-rule-syncer, observatorium/up, observatorium/operator, observatorium/opa-ams, observatorium/token-refresher