First off there is a helpful box to the right of the input on the ask a question page. It holds some of the most conman markdown syntax. If you click the link in the box called formatting help it will take you to a much more comprehensive list of the mark down syntax.
If you have just a few conmanly used lines you want to save I don't think there is any problem with putting them in your user profile, (I do).
You could use a external editor to write your question and answers (but for most question and answers it is overboard).
There are many good options available as online apps and as downloads.
If you just want to have a place to keep notes on the question page try one of these browser addons.
FireFox floatnotes, internote
chrome Note Anywhere (I have not used any of the browser extensions)