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

Start with the official SageMath reference manual and the sagemath/documentation repository linked in the issue. Review how DevDocs adds an external documentation set, then determine the integration entry point and verification steps. Done means the SageMath reference manual is available and usable in DevDocs.

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