Missing links and out-of-date systems overviews on documentation page

[michaelrsweet]

Version: 1.1.18
CUPS.org User: mike

The documentation page is missing a link to the STP and several of the recently released RFCs for IPP.

Also, the system overview section of each document still refers to the old GNU Ghostscript 5.50 RIP.

[michaelrsweet]

CUPS.org User: mike

Links updated.

[michaelrsweet]

CUPS.org User: mike

Docs updated.