Can I submit a non-research "article" on ArXiv?
See Is it possible to upload expository papers to arXiv? .

I have seen linear algebra notes on the arXiv, and also notes on other undergraduate-ish subjects (including some pretty good ones, e.g., http://arxiv.org/abs/1302.6946 and http://arxiv.org/abs/0805.0585 ). I absolutely don't mind them, and I don't think others do (although I would suggest using github for course notes due to the possibility of frequent corrections and updates). Expository papers for nonstandard material definitely fit well on the arXiv, and there are several; e.g., almost every chapter of the Tamari Memorial Festschrift has been posted on the arXiv.