This has over 30 votes, and is one of the top voted apps that is not yet integrated.
On hackernews, when slack pricing changed, zulip seemed to get the highest marks as product-wise as an alternative. Better still than mattermost.
Their docker is so nice, I was able to get it up and running on a bare metal server in five minutes. No joke.
Check out their demo site
"pd3f is an Open-source PDF text extraction pipeline that is self-hosted, local-first and Docker-based.
pd3f reconstructs the original continuous text with the help of machine learning."
If you have a static site like jekyll, hugo, github pages, etc. and you want to allow users to comment or submit user-generated content, staticman is the best solution.
Unlike Disqus which is third-party, owns your data, and injects a ton of garbage JS assets into your site, staticman is free, open-source, and lightweight.
The workflow is very cool. When the user submits something, staticman submits a pull request to your static sites git repo. If you approve the user content (e.g. a threaded comment), you merge the PR and it becomes live and static as part of your site.
Don't take my word for it. These people also love staticman: