ci: add PR CI quality gates & refine PR template

[mhoff]

Description

• ci: enforce CHANGELOG.md is updated
• ci: enforce PR TODOs are completed
• ci: introduce umbrella job for enforcing merge status checks with GitHub

Assignee

• The changes adhere to the contribution guidelines
• I have performed a self-review of my code
• My changes generate no new warnings (e.g. flake8/mypy/pytest/...) other than deprecations

Documentation

• README.md is up-to-date
• CHANGELOG.md is up-to-date
• Documentation (doc/source) is up-to-date
• Preview for docs looks fine
• Notebooks are up-to-date and functional
• Examples are up-to-date and functional (including compose & k8s)

Code Quality

• Patch test coverage > 95% and does not decrease
• New code uses correct & specific type hints

How did you verify that the changes work in practice?

• GHA

Reviewer

• The code is readable/maintainable and follows the contribution guidelines
• Documentation is up-to-date
• Tests (unit & acceptance) are meaningful and not over-mocked

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The rendered docs for this PR can be found here.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.16%. Comparing base (ce970b7) to head (1593ba2).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1011      +/-   ##
==========================================
- Coverage   93.16%   93.16%   -0.01%     
==========================================
  Files         234      234              
  Lines       16070    16086      +16     
==========================================
+ Hits        14972    14986      +14     
- Misses       1098     1100       +2     

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