Docs custom domain

[jamescdavis]

Turns out all we need to do here is set the root URL to ''.

After merging this PR, the steps for enabling the custom domain are:

1. re-deploy master (and update latest):
   ADDON_DOCS_UPDATE_LATEST=true ember deploy production
2. immediately enable the custom domain here by entering:
   ember-cli-typescript.com
3. update and re-deploy the octane-docs branch so it continues to work too:
   git checkout octane-docs && git merge master && ember deploy production

I've tested this on my fork and then removed the custom domain there so typed-ember can pick it up. DNS is already configured for github pages.

Once re-deployed & configured, docs will be available at https://ember-cli-typescript.com and https://typed-ember.github.io/ember-cli-typescript/ will redirect there.

Also, ember-cli-addon-docs has a bug, which has been fixed with ember-learn/ember-cli-addon-docs#412, but not released yet, so we need to pin to a sha to re-deploy and have our blue brand customization work. We're already doing this in the octane-docs branch. This will just bring it into master so we can re-deploy master too.

[chriskrycho]

💥