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.
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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.