Document and publish release process and release cadence for all Cortex project components

[alolita]

As a best practice, the project should add documentation for the release process and cadence for all project components.

We could also make sure release process documentation for all project repos is up-to-date.

See
https://github.com/cortexproject/cortex/blob/master/RELEASE.md
https://github.com/cortexproject/cortex-tools/blob/main/RELEASE.md
https://github.com/cortexproject/cortex-jsonnet/blob/main/RELEASE.md
https://github.com/cortexproject/auth-gateway/blob/main/RELEASE.md
https://github.com/cortexproject/promqlsmith - missing build instructions