APIv3: add hosted documentation URL to build API

[agjohnson]

For the case of PR builds, it could be helpful to provide the URL to the PR build in the Build APIv3 endpoint. Currently, you can only get the URLs of public versions from the Version/Project API.

Uses cases here might be for CI integration with Read the Docs, and we'll likely want an asynchronous version of the resolver for this URL for keeping the new UI fast.