Add Mappings for NIST SSDF

See https://csrc.nist.gov/projects/ssdf.

For example MASWE-0041 

```yaml
mappings:
    nist-ssdf: [PS.3.2]
```

For maswe.instructions.md:


```md
  - **nist-ssdf**: One or more practice IDs from the [NIST Secure Software Development Framework (SP 800-218)](https://csrc.nist.gov/projects/ssdf). Optional.
```