docs: restore accidentally deleted defaults.md

docs/internals/defaults.md

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+# Defaults
+
+- multiple server nodes
+  - by default, when `--server` > 1 and no `--datastore-x` option is set, the first server node (server-0) will be the initializing server node
+    - the initializing server node will have the `--cluster-init` flag appended
+    - all other server nodes will refer to the initializing server node via `--server https://<init-node>:6443`
+- API-Ports
+  - by default, we expose the API-Port (`6443`) by forwarding traffic from the default server loadbalancer (nginx container) to the server node(s)
+  - port `6443` of the loadbalancer is then mapped to a specific (`--api-port` flag) or a random (default) port on the host system
+- kubeconfig
+  - if `--kubeconfig-update-default` is set, we use the default loading rules to get the default kubeconfig:
+    - First: kubeconfig specified via the KUBECONFIG environment variable (error out if multiple are specified)
+    - Second: default kubeconfig in home directory (e.g. `$HOME/.kube/config`)
+- Networking
+  - [by default, k3d creates a new (docker) network for every cluster](./networking)