intro: add curl example to add new video to download queue

[erentar]

No description provided.

[bbilly1]

As this is an example for adding videos to the queue, that would fit better on the corresponding page that documents adding to the queue. Then for styling, there is already an example with "Example" here, you can mimic that.

Then no need to over complicate things with variables and stuff, maybe just consolidate everything into one curl command. Then best to be consistent with the other examples, e.g. something like that:

curl -XPOST -v /api/download/ \
    -H "Authorization: Token xxxxxxxxxx" \
    -H 'Content-Type: application/json' \
    -d '{"data":[{"youtube_id": "<video-id>", "status":"pending"}]}'

You can't assume the domain, so best to be obvious that the user will have to add that. Same for the video ID.