Of course Github is just an example but you can pretty much regex any URL and further filter out anything in order to get the apk link with it. So depending on your level of privacy requirement and trusted sources, you can skip all the centralized ones and build your own list of sources.
Well if you happen to find a good app that does what you want on desktop, then you can complement it on Android with Markor , it’s “markdown native” and it has nice extra options like adding list elements, quick formatting etc. All based on markdown, can be easily switched between editor mode and preview(markdown rendered) mode.