Tiuku

If markdown fulfills your formatting needs, then there's no beating it in terms of focus and simplicity. Use whatever text editor you like. My recommendation would be Kate. It supports previewing the rendered document in side by side view.

I'm a shell user too, but as a programming language I would rate Bash utter garbage. Fine for little piping but for longer scripts I will be reaching for Haskell.

There's also Brewtarget.