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?
AssemblyAI— Speech-to-Text API with superhuman accuracy
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.