Add documentation on using admin-ui through fidesops server

[eastandwestwind]

Docs Update Description

Due to changes in #625, we'll need to add docs for how to nav to the admin ui when the fidesops server is up and running.

[TheAndrewJackson]

We'll also want to add some docs describing how to display the admin ui from within the fidesops server while developing the backend. By default the docker-compose.yml config will "remove" the ui files that get built into the docker image by mounting the local fidesops directory and display a placeholder instead. Running npm run prod-export from within the clients/admin-ui directory will build and place the admin ui within the correct fidesops location so it will show up as expected when developing the backend.