I am not familiar with MathJax format. Can I draw simple geometric shapes in my question, without uploading a picture?
No, not really. MathJax is primarily built to display mathematical equations, not mathematical pictures. They don't explicitly state it's not supported, but they do state the following about their Core Goals:
Our key design goals are:
- High-quality display of mathematics notation in all browsers.
Note that even richer markup languages like TeX have to rely on packages/plugins like TikZ to draw pictures.
Now, it might be possible to 'abuse' certain MathJax features to output some kind of drawing. However, it's a lot easier to just pick a tool which is meant for drawing. Some helpful resources can be found in the links mentioned under this question: Is there a standard drawing tool used for asking or answering questions?