api: docs: start documenting nodegroup and tree

[ffromani]

doc-only change

[openshift-ci]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ffromani

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [ffromani]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[Tal-or]

/lgtm

[openshift-ci]

@ffromani: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
ci/prow/ci-e2e	71cb55e	link	true	/test ci-e2e
ci/prow/ci-install-e2e	71cb55e	link	true	/test ci-install-e2e

Full PR test history. Your PR dashboard.

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes-sigs/prow repository. I understand the commands that are listed here.

[ffromani]

a doc only change can't break anything, and the CI is currently failing for known reasons