Document how user can clean up and reuse their cluster

It is helpful to be able to reuse a cluster than you have previously installed applications on. We plan on making this easy in https://gitlab.com/gitlab-org/gitlab-ce/issues/58022.

In order to help users prior to implementing this feature, we can update our documentation to include the steps a user must take in order to reuse their cluster. This will include removing the gitlab-managed-apps namespace. We can also include a cleanup script.

The documentation should note that we do not support this workflow, so it is possible that any new changes to our integration that creates other resources not covered by the cleanup script will again cause problems for users.

We should also link to https://gitlab.com/gitlab-org/gitlab-ce/issues/58022 as the official solution we are working on.