Move API docs to api.dartino.org

[mit-mit]

We should move our API docs over to the correct subsite https://api.dartino.org rather than the current temporary location on github pages at https://dartino.github.io/api/.

[mit-mit]

Task list:

• [x] Step 1: The firebase hosting project api-dartino-org has been configured for the custom URL api.dartino.org
• [x] Step 2: We now have a new repo, api.dartino.org, which through a TravisCI job pushes to Firebase
• [ ] Step 3: Update promote.py to push to the new repo, https://github.com/dartino/api.dartino.org. It should place the docs in the public directory in the master branch
• [ ] Step 4: Update the API link in the main website menu
• [ ] Step 5: Deprecate old repo, https://github.com/dartino/api

[mit-mit]

Steps 1 and 2 completed.

[mit-mit]

Step 3 CL: https://codereview.chromium.org/2165663002