The original version of the FRED documentation can be found here (Fred Docs); however, the BSG-themed documentation (linked above) has been updated for FREDOpen.
However, the BSG-themed version (Diaspora FREDdocs) isn't complete yet, and a complete version won't be publicly available until the Diaspora R1 release. The Diaspora FREDdocs will eventually be ported to FS2.
If you're just starting, your best bet is to look at the retail FREDdocs, which can be downloaded from the link above or viewed online here
When you look at the FREDdocs, be sure to do the full walkthrough
. It will take you a couple of hours to finish, but it is absolutely
worth the time. This Q&A
explains why doing the walkthrough is so important.
The FRED Portal
on the FreeSpace Wiki is also worth a look.