Document how to roll out and roll back applications

[juliogreff]

Rolling out a new version of an application is very implicit in our documentation, an user needs to be familiar enough with kubernetes to realize that you need to change the application spec to trigger a new release.

Similarly, it's not explicit that to go back to a previous release, one needs to either delete releases from the top, or re-apply the application spec to generate a new release that matches an old one.