docs: add policy types and step constraints to autoscale page

pages/features/autoscale.mdx

@@ -196,6 +196,71 @@ You can use the `watch` tool to see if you catch the instance changing state fro
 watch --color -n 0.5 kraft cloud instance list
 ```
 
+## Policy types
+
+Four autoscale policy types are available.
+A service can have more than one policy active at the same time.
+
+### Step policy
+
+The step policy scales instances based on metric thresholds.
+You define up to **4 steps**, each specifying a lower bound, upper bound, and the scaling change to apply when the metric falls in that range.
+
+Order steps by lower bound with no gaps between them and no overlaps.
+
+```bash title=""
+kraft cloud scale add <service-name> \
+  --name my-step-policy \
+  --type step \
+  --metric cpu \
+  --adjustment-type change \
+  --step 600:800/2 \
+  --step 800:/4
+```
+
+### On-demand policy
+
+The on-demand policy creates a new instance as soon as a request arrives with no available instances to handle it.
+This minimises latency at the cost of cold starts.
+
+### Create policy
+
+The create policy pre-warms instances ahead of demand, keeping a pool of ready instances available.
+Use it for workloads where cold-start latency can't occur.
+
+### Idle policy
+
+The idle policy scales in (removes instances) when the service has been idle—receiving no requests—for a configurable period.
+
+## Metrics
+
+The following metrics can drive a step policy:
+
+| Metric | Description |
+|--------|-------------|
+| `cpu` | CPU utilization in millicores |
+| `inflight_reqs` | Number of requests the platform is processing across all instances |
+| `reqs_per_sec` | Request throughput in requests per second |
+
+## Scale change types
+
+Step policies support three scaling change types:
+
+| Type | Description |
+|------|-------------|
+| `change` | Change the instance count by the specified value (positive to scale out, negative to scale in) |
+| `exact` | Set the instance count to exactly the specified value |
+| `percent` | Change the instance count by the specified percentage of the current count |
+
+## Step policy constraints
+
+- Maximum **4 steps** per step policy
+- **Sort** steps by lower bound, ascending
+- **Contiguous**: no gap between the upper bound of one step and the lower bound of the next
+- Steps must not **overlap**
+- Each step must have `lower_bound < upper_bound`
+- A step can't span the entire range (`lower_bound = -∞` and `upper_bound = +∞` simultaneously)
+
 ## Learn more
 
 * The `kraft cloud` [command-line tool reference](/cli/), and in particular the [scale](/cli/scale) subcommand.