JP-3554: Add MSA meta data file docs

[hbushouse]

Resolves JP-3554

Closes #8314

This PR adds detailed docs for the contents and usage of the NIRSpec MSA metadata file, which is used during Spec2 processing of NRS MOS exposures. In addition to a new page describing the MSA metadata file itself, I've also added some updates to the assign_wcs step docs that reference the MSA metadata file and just make a few minor updates/fixes to the assign_wcs docs themselves.

Checklist for maintainers

• added entry in CHANGES.rst within the relevant release section
• updated or added relevant tests
• updated relevant documentation
• added relevant milestone
• added relevant label(s)
• ran regression tests, post a link to the Jenkins job below.
  How to run regression tests on a PR
• Make sure the JIRA ticket is resolved properly

[hbushouse]

Link to as-built RTD page: https://jwst-pipeline--8399.org.readthedocs.build/en/8399/jwst/data_products/msa_metadata.html

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 75.77%. Comparing base (2fb073e) to head (17abdb4).
Report is 19 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #8399      +/-   ##
==========================================
+ Coverage   75.31%   75.77%   +0.46%     
==========================================
  Files         474      476       +2     
  Lines       38965    39442     +477     
==========================================
+ Hits        29345    29887     +542     
+ Misses       9620     9555      -65     

Flag	Coverage Δ		*Carryforward flag
nightly	77.63% <ø> (+0.29%)	⬆️	Carriedforward from 1d8f0fe

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[emolter]

LGTM