testing/fuzz: improve documentation on how to fuzz suricata
We have a readme page on how to run the fuzzers, and an unfinished devguide page on how to test
suricata with fuzzers.
This task is meant to cover updating the readme file and complete a v1 for the testing page.
This is a follow-up of the work done in https://github.com/OISF/suricata/pull/6569