[docs] fix broken links on network and describe PFNN on spec

[shyba]

Description

Hi, I was reading the docs and spec when I found some broken links and what looks like an inconsistency. The specification states that there is no PFNN, but documentation shows how to connect a PFN to another PFN. I added a small description for the network formed by connecting PFNs to other PFNs and fixed the broken links.

Test Plan

PFNN description is clear. Previously broken links now point to the correct pages (maybe I got the wrong one?)

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This change is