Flesh out step types docs

[applejag]

The step types docs are very basic at the moment. Using solely a code block to describe everything.

This is fine for us internally, but does not show a greater grasp over how these step types work.

Suggested convention:

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# Container step

Short description of what it does.

Longer description of how it works.

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## Minimal example

```yaml
myStage:
  myStep:
    container:
      image: alpine:latest
      cmds:
        - echo 'hello world'
```

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## Parameters

### `image`

Type: `string`

Short description of what it does

```yaml
image: alpine:latest
```

### `cmds`

Type: `string array`

Short description of what it does

```yaml
cmds:
  - echo 'hello world'
```

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## Full example

```yaml
myStage:
  myStep:
    container:
      image: alpine:latest
      cmds:
        - echo 'hello world'
      os: linux/windows
      shell: /bin/sh
      secretName: mysecret
      serviceAccount: default
      certificatesMountPath: /usr/local/share/ca-certificates
```

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Preview: