Update Toolbar docs to include how to authenticate on the frontend

[blakewilson]

From the toolbar perspective, you can only authenticate via "previewing" a post. Let's add to the docs how to create a frontend login page to facilitate login and easier access to the toolbar.

See the toolbar example for how login works here: https://github.com/wpengine/faustjs/blob/canary/examples/next/custom-toolbar/wp-templates/front-page.js