Add some code example in documents

[peterzcy]

Venus simulator is a good resource for learning RISC-V language. As I use it for programming, I have to look up at the documents for what instructions or assembler derectives it supports, then search online for examples for that instruction. If there were some example code in the document, it will be time-saving. And we, who are taking a related course about RISC-V, are willing to finish that work.