docs: add page about k3d concepts (incl. nodefilters) (#888)

docs/design/.pages

@@ -1,5 +1,6 @@
 title: Design
 nav:
   - project.md
+  - concepts.md
   - defaults.md
   - networking.md

docs/design/concepts.md

@@ -0,0 +1,33 @@
+# Concepts
+
+## Nodefilters
+
+### About
+
+Nodefilters are a concept in k3d to specify which nodes of a newly created cluster a condition or setting should apply to.
+
+### Syntax
+
+The overall syntax is `@<group>:<subset>[:<suffix]`.
+
+- `@` denotes the start of a nodefilter in a k3d flag value
+- `<group>` denotes the node group you want to filter in
+  - one of `server`, `servers`, `agent`, `agents`, `loadbalancer`, `all`
+    - note, that `all` also includes the cluster-external server loadbalancer (`k3d-proxy` container)
+- `<subset>` denotes the subset of the chosen group you want to apply the flag to
+  - wildcard `*`: all nodes in that group
+  - index, e.g. `0`: only the first node of that group
+  - list, e.g. `1,3,5`: nodes 1, 3 and 5 of that group
+  - range, e.g. `2-4`: nodes 2 to 4 of that group
+- `<suffix>` (optional) can hold some flag specific configuration
+  - e.g. for the `--port` flag this could be `direct` or `proxy` (default) to configure the way of exposing ports
+
+### Example
+
+- Problem: You want to have Nginx as your ingress controller, but by default, K3s deploys Traefik.
+- Solution: Disabling the default Traefik deployment using K3s' `--disable=traefik` flag.
+- Note: It's enough to do this on the first (initializing) server node.
+- How-To: `k3d cluster create notraefik --k3s-arg="--disable=traefik@server:0"`
+  - Looking at `--k3s-arg="--disable=traefik@server:0"`, everything after the `@` sign is part of the nodefilter.
+    - `server` is the node group: server nodes
+    - after the `:` follows the subset, which in this case is the index `0`: the first server node to be created (`k3d-notraefik-server-0`)