use towncrier to handle change log entries and add instructions to new pull requests

[zacharyburnett]

new checklist

Tasks

• update or add relevant tests
• update relevant docstrings and / or docs/ page
• Does this PR change user-facing code / API? (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 regression tests with this branch installed ("git+https://github.com/<fork>/stpipe@<branch>")
    • jwst regression test
    • romancal regression test

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 73.29%. Comparing base (5efae61) to head (284d055).
Report is 29 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #187   +/-   ##
=======================================
  Coverage   73.29%   73.29%           
=======================================
  Files          25       25           
  Lines        1917     1917           
=======================================
  Hits         1405     1405           
  Misses        512      512           

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[braingram]

LGTM.