At some point, You understand it's not enough to use GitHub/BitBucket for issue management and document sharing. So You start looking for something good and preferably free.
There are plenty of free different solutions but in fact, if GitHub/GitLab/BitBucket built-in issue tracker is not ok for you, then try Jira Cloud - it's free for less than 10 users on the project.
You could check Trello, Asana, or Redmine as an alternative or browse something else but Jira is standard in software development. Plenty of big companies use it so for you it will be a good chance to learn something useful on commercial projects.
There are much more alternatives for document sharing on the internet
You could start with Google Docs - it's simple as that. Mostly it's enough
If you need something advanced - check Confluence. Confluence as well as Jira is developed by Atlassian and is standard in software development too. Free for 10 users.
For those who love beautiful tools I recommend Notion - amazingly nice user interface but a tricky and requires learning. Notion has a free plan for 5 users.