As @DTLow says, start with a vault folder in obsidian and then index this to Devonthink. It is exactly what I do for my ‘ideas’.
One word of caution. If you need to create sub folders/groups and move things around, do it in DT not obsidian or you can get lots of ‘missing link’ to documents. (They are easy enough to resolve, but better not to have the hassle). DT will apply any changes back to the obsidian folder. Any new MD documents created in either DT or obsidian will show up in both.
The vault can even be in obsidian iCloud so you get it on all devices.
Here is an example where I maintain a markdown ‘cook book’ in DT but use Obsidian for display on my iPad when cooking.
