Add Sagemath docs

# Feature request

Add the reference manual of [SageMath](https://www.sagemath.org/), a python-based open-source math software system.

## Summary

Official reference manual: https://doc.sagemath.org/html/en/reference/index.html
The entire site is also available at [sagemath/documentation](https://github.com/sagemath/documentation).

## Examples