DOC: update installation instructions
DOC: update installation instructions

add centos8
drop ubuntu 19.10
wording changes to make it clearer
drop unnessary APT key instructions, already part of the SOO page

certtools/intelmq,aaronkaplan/intelmq,certtools/intelmq,aaronkaplan/intelmq,aaronkaplan/intelmq,certtools/intelmq