Hi @jmillerokc ! Thanks for tagging me @TitiCuisset , documentation is one of my favorite topics, I’m actually teaching a class on it right now!
My 1 tip is to use a system your team is already using daily so you don’t have to teach them a new tool IN ADDITION to teaching them new habits about creating, using, and updating documentation.
If you’re more concerned about documenting for yourself right now, getting the first draft of all the docunentation done, my advice is use the tool that’s easiest for you. You can always delegate/hire someone later to move it into a different system but it would be harder to delegate or hire anyone to do the first draft of it -- for someone else to get that information out of your head!
So as you mentioned, the HubSpot knowledge base would be a good place to store documentation, if your team is using HubSpot daily already, you have reasons to purchase Service Hub it in addition to the knowledge base (HubSpot has a lot of material to help you make a case for that, if needed!). You can use it as the main library and link articles to, or embed, flow charts and other items if an article format isn’t as useful.
If you currently use a project management system, ideally one shared across the team or company, that can be a great place to store documentation since ideally everyone is already in that tool daily.
If you use Google docs as a team and people are used to navigating to find information in there, and can find what they need, that can be a good option at least to start with, to build out the documents and get people in the habit of using and updating.
There are tools like Notion or Guru that integrate with Slack, if your team is a big user of Slack as your “home base” for work during your day…I just caution people to not get too wrapped up in spending weeks on researching documentation tools when its a better use of time to spend those weeks completing your version 1.0 of documentation in whatever tool is easiest for you and your team. Using what you have now. Since tool research can be much more fun (and distracting) than doing the documentation 
Resources:
Recent HubSpot admin HUG with my previous boss Nicole about our documentation learnings
Talk I did at world certification week
I may have taken over this podcast topic to talk about documentation haha
4 part blog series on HubSpot community blog, starts here
Class