The best alternatives to HackMD are GitHub, Code.gov, and Quirk 2.0. If these 3 options don't work for you, we've listed a few more alternatives below.
What do you think of HackMD?
Snowflake - BUILD— The Free Dev Conference for AI & Apps on Nov. 12-14
Quirk is an open source cognitive behavioral therapy (CBT) app for iOS and Android. Unlike most apps, it's accessible enough to be used for your day-to-day problems like imposter syndrome, interview anxiety, breakups, fears of flying and more.