The admin client is pretty much intended as a client deliverable. i.e. it makes it easier for non-technical users to be able to maintain their site after delivery.
As an experienced developer, you should be able to develop the site without the admin, in fact you'll probably find it really just gets in the way of developing the site. You may find it useful for populating some content, but that's really about it. You will have to be knee deep in Twig for HTML layouts, and CSS etc. So you really need to understand how to work at the file level. The learn docs assume that is the case and provide lots of information regarding this.
We are now working on documentation for the admin plugin, but as I said, this is not something that you should rely on during development.
Installation of the admin is covered here: http://getgrav.org/blog/beta-admin-plugin-available
But you should really have your site up and running before even attempting to install it.
If you ne ed more detailed help, I strongly suggest hoping on the Gitter chat room (link in top right corner of this site) and chatting with us live.