Documenting a whole project

Hi, so I'm not exactly sure if this is the best kind of question for this sub, but I'm going to ask it anyway. So I'm working on developing a rather large software solution with a tiny team. We are pretty much a startup and as such I'm having to wear a lot of hats in this project, and it's been really fun for the most part, but I'm thinking I want to bring in more structure to our development and was looking into the best practices when documenting the project and keeping track of tasks. Things like Trello were the first to come to mind, and they work, but I would like to come up with a document describing all the features of the software, big and small, that have been added and still need to be added, so that then perhaps from there we can use that document internally with every developer as a reference and, for example, making Trello cards would be much easier when we have the tasks and features already laid out and specified. I'm just not sure what kind of structure the document should have, I am pretty inexperienced when it comes to PM and this kind of all-encompassing detailed documentation. Any tips on how I can go about figuring out the structure or maybe there are some tools that can help with this task?