docs: add grove intro docs

Description

Documentation to make the introduction to Grove/Kubernetes easier.

Testing instructions

• Run mkdocs serve
• Read the generated doc, http://localhost:8000/user-guides/kubernetes-for-beginners/

Checklist

If any of the items below is not applicable, do not remove them, but put a check in it.

• All providers include the new feature/change
• All affected providers can provision new clusters
• Unit tests are added/updated
• Documentation is added/updated
• The TOOLS_CONTAINER_IMAGE_VERSION in ci_vars.yml is updated
• The grove-template repository is updated