The On-Topic page says:
There are certain subjects that, while still being on-topic here, you can get better response on our sister sites: [...]
I suggest adding to this list:
- teaching mathematics and the process of learning mathematics: Mathematics Educators
- signal, image and video processing: Signal Processing
Also, two corrections for the line
Research-level mathematics — Math Overflow
"MathOverflow" should be written as one word (unlike Stack Overflow). And the name should be a hyperlink.