Write Kubernetes back-end documentation

[chrisdoherty4]

Having introduced the Kubernetes back-end to the Tinkerbell stack we need to highlight in documentation that it exists.

The exact strategy/extent of the documentation is still under discussion. Options include:

1. Document all the details on the docs website via this repository.
2. Document what back-ends exist and point to the sandbox that details specifics in how to deploy specific back-ends.

Related

• Document the Kubernetes back-end deployment playground#147

[chrisdoherty4]

Currently, we're leaning into option 2. Other options suggestions welcome.