chore(ci): deploy to `docs.angularjs.org` from within the corresponding directory

Previously, in order to deploy to Firebase from
`scripts/docs.angularjs.org-firebase/`, we had to copy the
`firebase.json` file to the repository root and adjust the contained
paths accordingly.

By running the `firebase` CLI directly (instead of via `yarn`), we are
able to deploy from `docs.angularjs.org-firebase/` directly. This
simplifies the deployment (and local testing) process and paves the way
for also deploying from `code.angularjs.org-firebase/` in a subsequent
commit.