Undersampling docs

[jotaylor]

This PR updates the undersampling_correction step description to fix some grammatical typos and improve the readability. There are no associated issues or Jira issues. However, one thing I didn't know how to do was link to a subsection of another page. So the words "charge migration" in the 3rd sentence should link to the Charge Migration subsection of the ramp_fitting step: https://jwst-pipeline.readthedocs.io/en/latest/jwst/saturation/description.html#charge-migration
Can someone please tell me how to do so?

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

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (3c3edba) 77.48% compared to head (a60423e) 77.48%.

❗ Current head a60423e differs from pull request most recent head 09d3433. Consider uploading reports for the commit 09d3433 to get more accurate results

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #7589   +/-   ##
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  Coverage   77.48%   77.48%           
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  Files         456      456           
  Lines       36591    36591           
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  Hits        28354    28354           
  Misses       8237     8237           

Flag	Coverage Δ		*Carryforward flag
nightly	77.54% <ø> (ø)		Carriedforward from b728a57

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Impacted Files	Coverage Δ
jwst/tweakreg/utils.py	93.33% <100.00%> (ø)

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[hbushouse]

@jotaylor Regarding the reference to charge migration (in the saturation docs), the most robust way is to create an explicit target in the saturation page and then reference it from the undersampling page. For example, at https://github.com/spacetelescope/jwst/blob/master/docs/jwst/saturation/description.rst?plain=1#L36 add a line containing

.. _charge_migration:

(yes, all the dots, underscores, and colon are required). Then in the undersampling page insert a reference/link to that target using

:ref:"charge_migration"

(again, all the punctuation is required). But NOTE that the double quotes around charge_migration should actually be a single back-tic, I just can't get github to render the back-tics properly, because they're also used for formatting! Just grep some of the other doc pages for the string ":ref:" and you'll see the formatting.

[hbushouse]

Looks good. Just a few minor suggestions.

Oh, you also need to add an entry to the CHANGES.rst (change log) file at the top level of the repo. But an entry in the "documentation" subsection and follow existing entries for the proper formatting, including the PR number (7589).

[jotaylor]

Thanks for the comments @hbushouse ! I think I've addressed them all, but please let me know if anything else needs updated.

[hbushouse]

Couple of errors in syntax for the ramp_fit links need fixing.

[hbushouse]

Looks good. All of the links now work correctly in the new docs.