[WIP] Adding Documentation for Enabling External Authentication - Depends on External auth3 pull request.

Adding ManageIQ/Guides for installing and enabling
external authentication via IPA.

Depends on the External auth3 Pull Request https://github.com/ManageIQ/manageiq/pull/32


Is there a consistent way to show "navigation"?

I always use ` -> ` with spaces around the operator...e.g. `Configure -> Configuration -> Zone -> Server -> NTP`