[Fleet]: Create API key step is missing for standalone agent

**Description:**
`Create API key` step is missing from the standalone agent policy documentation. Users following this guide cannot complete the setup because the instructions for generating the required API key are not included.

**Link:**
https://www.elastic.co/docs/reference/fleet/create-standalone-agent-policy

**Steps to Reproduce**

1. Navigate to the [Create a standalone agent policy](https://www.elastic.co/docs/reference/fleet/create-standalone-agent-policy) documentation page.
2. Follow the instructions for setting up a standalone agent.
3. Observe that no step covers creating the API key required for the agent.

**Screen-shot:**

**Expected Result**
- Create API key step should be available for standalone agent