This meme is relevant here again. 
Even if I was just one person I would have a shared repository of test things (cases, charters, notes). A place that outlives me if I leave and help me remember stuff over time. Even more when there are two people. github, wiki, excel, .. doesn’t matter as such.
There is some sense in having this close to the dev’s stuff. So if they are in jira - or devops or SAP Solman over what ever, go there. align with them. Perhaps over time the test artifacts will require it’s own place - deal with that when you get to it.
When you start using any prefabricated system (eg test casemanagement tool) it will shape the way you do your work. Be aware of this in the tool selection. Check @TheyWrestleTest s work on this topic btw.
Some of the contexts I work in have it as a contract or otherwise legal requirement, so that’s a whole other ballgame. See Are formal test plans and lack of shifting left red flags?
In conclusion, considering your context I would start simple as @sebastian_solidwork also suggests 