Add example of fetching playback URL for asset imported from dStorage gateway using dStorage ID

[yondonfu]

The docs currently contain an example of creating and watching an asset. This example relies on the Studio direct upload feature via a drag & drop zone.

Once livepeer/studio#1362 is implemented, we should add an example of "importing and watching an asset" that showcases:

• Specifying a dStorage import URL (i.e. IPFS, Arweave gateway)
• Importing the asset from the gateway to Studio
• Watching the asset playback after it has been imported and processed

The code walkthrough should showcase:

• The use of a CID to playback the imported asset if it was from an IPFS gateway
• The use of an Arweave tx ID to playback the imported asset if it was from an Arweave gateway