DOC: Code Standards

## Description
Generate documentation to describe the SkyPy code standards.
Update readme accordingly.

## Output
- New documentation page with the SkyPy Code Standards under the section [Developer Documentation]( https://skypy.readthedocs.io/en/stable/#developer-documentation)

## References
- [Astropy Code Standards](https://docs.astropy.org/en/latest/development/codeguide.html)

