Refactor settings pattern

[benjaoming]

This PR implements a new settings pattern that's friendly to tests using Django's override_settings decorator. It's also friendly to our documentation, such that we can simply render the docstrings of django_nyt.settings.defaults in order to have a nice Configuration section in our documentation.

• [x] Fixes #124

• [x] Changes via suggestions and inputs from the Fediverse, including desired best-practice:

  • Support for IDE auto-suggested settings names
  • Support for type hints
  • Easily list available settings in Sphinx autodoc documentation
  • Be lazy and support @override_settings
  • Dataclasses: In this pattern, we use dataclasses without benefiting much, and maybe it's a bit YAGNI. But it seems like the right approach, especially when using a single dictionary to hold the entire app's settings.

The reST needed to include the final documentation for the app settings:

.. automodule:: django_nyt.conf
   :noindex:

   .. autoclass:: AppSettings()
      :members:

Documentation page BEFORE:

Documentation page AFTER:

[codecov[bot]]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (ae58eb8) 74.30% compared to head (bcf1e7a) 74.90%.

Files	Patch %	Lines
django_nyt/consumers.py	0.00%	1 Missing :warning:
django_nyt/subscribers.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #127 +/- ## ========================================== + Coverage 74.30% 74.90% +0.60% ========================================== Files 23 23 Lines 541 554 +13 ========================================== + Hits 402 415 +13 Misses 139 139 ```

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[benjaoming]

I'm gonna merge this now, not as in "this is perfect", but as in "this is the end of Chapter 1", but look forward to a sequel! The plot is unknown, but I have a good feeling we'll see some action with Django AppConfig.

Blog post: https://overtag.dk/v2/blog/a-settings-pattern-for-reusable-django-apps/

Edit: The other reason is that incidentally this PR also fixes the Read the Docs deployment that had started using the "default" Sphinx theme :scream: