Suricata

Our userguide has a section for installing from Ubuntu, but there are some details about PPA options
that are only present in our wiki: https://redmine.openinfosecfoundation.org/projects/suricata/wiki/Ubuntu_Installation_-_Personal_Package_Archives_%28PPA%29
This task comprises two steps:
- update userguide
- update suricata.io/downloads to link to updated userguide section
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Make sure to add a note (or have a backport ticket) distinguishing from suricata-stable for the latest version, and suricata-6.0 for Suri 6
(packages details at https://launchpad.net/~oisf), as pointed out on Discord.