Add schema types description to docs (and misc cleanup)

[braingram]

This is far from a complete cleanup but addresses a few docs issues:

• Remove a redundant list of datamodels in the docs. https://stdatamodels.readthedocs.io/en/latest/jwst/datamodels/index.html#module-jwst.datamodels contains a list that is automatically updated and links to the models. This PR removes the static list (which otherwise requires manual updates): https://stdatamodels.readthedocs.io/en/latest/jwst/datamodels/attributes.html#list-of-current-models
• Updates README to make it clear this package is for JWST (not generally for STScI missions)
• Adds a brief description of the schema types in this package (data model, reference file, transform)

I also included a test that checks data model schemas to verify they either reference:

• referencefile.schema (for reference file schemas)
• core.schema (for data model schemas)

There are 2 exceptions (hard-coded in the test):

• slitdata.schema
• extract1dimage.schema

I think these both define extra extensions so are sort of partial datamodels. Perhaps this is worth an issue to document that these are different. It may make sense to add another category to the documentation.

Link to updated docs: https://stdatamodels--338.org.readthedocs.build/en/338/jwst/datamodels/schemas.html

As this touches no code (outside of the added unit test) no regression tests were run.

Tasks

• update or add relevant tests
• update relevant docstrings and / or docs/ page
• Does this PR change any API used downstream? (if not, label with no-changelog-entry-needed)
  • write news fragment(s) in changes/: echo "changed something" > changes/<PR#>.<changetype>.rst (see below for change types)
  • run jwst regression tests with this branch installed ("git+https://github.com/<fork>/stdatamodels@<branch>")

news fragment change types...

• changes/<PR#>.feature.rst: new feature
• changes/<PR#>.bugfix.rst: fixes an issue
• changes/<PR#>.doc.rst: documentation change
• changes/<PR#>.removal.rst: deprecation or removal of public API
• changes/<PR#>.misc.rst: infrastructure or miscellaneous change

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.51%. Comparing base (73c887c) to head (46a8519).
Report is 64 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #338   +/-   ##
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  Coverage   67.51%   67.51%           
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  Files         114      114           
  Lines        5913     5913           
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  Hits         3992     3992           
  Misses       1921     1921           

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[tapastro]

LGTM. I do wish there was "another way", wrt. behavior of MultiSlitModels and MultiSpecModels - their listed contents can be instanced into a SlitModel/SpecModel, but due to the nature of of stdatamodels are instead ObjectNodes while in the parent datamodels slits/spec listing.