Update runbooks / architecture diagrams for sharded thanos ruler

As part of &1068 (closed), we need to make sure we update the relevant areas of our Thanos documentation for how the sharded ruler works and is intended to work in the future.

Demo with the team the week of Aug 14th.

Updates to:

1. https://gitlab.com/gitlab-com/runbooks/-/blob/master/docs/thanos/architecture.md
2. Pull out readiness areas that should be in runbooks - https://gitlab.com/gitlab-com/gl-infra/readiness/-/blob/master/thanos/overview.md