Add documentation for stnode functionality

[WilliamJamieson]

This PR extends the docs so that they have a basic description of the stnode module

Checklist

• [x] Added entry in CHANGES.rst under the corresponding subsection
• [ ] updated relevant tests
• [ ] updated relevant documentation
• [ ] Passed romancal regression testing on Jenkins / PLWishMaster. Link: https://plwishmaster.stsci.edu:8081/job/RT/job/romancal/XXX/

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.55%. Comparing base (087a60d) to head (def790b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #316 +/- ## ========================================== - Coverage 97.56% 97.55% -0.01% ========================================== Files 30 30 Lines 2788 2786 -2 ========================================== - Hits 2720 2718 -2 Misses 68 68 ```

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[schlafly]

Thanks, those are helpful additional docs.