docs: documents Steps API.

docs/steps.md

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+# Steps
+
+Imposter can perform multiple operations when it receives a request. These are called Steps. You can add multiple steps to a resource, calling external systems, running scripts and using [stores](./stores.md). Steps can use the output of other steps, as well as data from the request, or stores. The response can use the outputs of steps, such as in a [response template](./templates.md).
+
+## Step Types
+
+Imposter supports the following step types:
+
+- Execute a script
+- Make an HTTP(S) request to a server
+
+> **Note**
+> Steps are executed in the order they are defined in the configuration file.
+
+### Simple example
+
+Here is an example with a single step:
+
+```yaml
+# ...part of your configuration file
+
+resources:
+- path: /example
+  method: GET
+  steps:
+    - type: script
+      script: |
+        console.log('Hello World!');
+```
+
+### Multiple steps
+
+Here is an example of a configuration file with multiple steps:
+
+```yaml
+# ...part of your configuration file
+
+resources:
+- path: /example
+  method: GET
+  steps:
+
+  # the first step will send an HTTP request to example.com
+  # and store its response status code and body in the 'current' object
+  - type: remote
+    url: http://example.com
+    method: GET
+    capture:
+      statusCode:
+        expression: "${remote.response.statusCode}"
+      responseBody:
+        expression: "${remote.response.body}"
+
+  # the second step will log the status code of the response
+  - type: script
+    script: |
+      console.log('Remote HTTP response status code: ' + current.statusCode);
+  
+  response:
+    content: "${current.responseBody}"
+```