Add documentation on how to add GitLab-managed-apps to the Kubernetes integration

Problem to solve

As more GitLab-managed-apps are added outside of the Configure:Orchestration group, users will need information on how to achieve this.

Proposal

• Record video outlining how to add an application
• Update documentation at https://docs.gitlab.com/ee/development/kubernetes.html

Other links/references

• MR adding elastic to GitLab-managed-apps !18015 (merged)