[webrtc-pc] RTCDataChannel section structure

lgrahl has just created a new issue for https://github.com/w3c/webrtc-pc:

== RTCDataChannel section structure ==
I think the `RTCDataChannel` section could use some subsections to make it easier to read and find relevant parts. No changes to the description, just adding headers:

```
  6.2 RTCDataChannel
    6.2.1 Create an instance
    6.2.2 Announce as open
    6.2.3 Announce an instance
    6.2.4 Closing procedure
    6.2.5 Announce as closed
    6.2.6 Error on creation
```

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Please view or discuss this issue at https://github.com/w3c/webrtc-pc/issues/1847 using your GitHub account