doc: add admin api doc

Continuing on my feedback above, the following is cleaner as:

Example Request:

```
POST http://localhost:2379/v2/admin/members/

[{"PeerURLs":["http://10.0.0.10:2379"]}]
```

Example Response:

```
HTTP/1.1 200 OK

[
        {
            "id":"3777296169",
            "PeerURLs":[
                "http://10.0.0.10:2379"
            ],
        },
    ]
```