Add a locale implementation guide

[user01010111]

Closes #983.

Adds a locale contributor checklist covering aliases, relative wrappers, timeframe forms, month/day indexing, formatting overrides and the existing test fixtures. Explains the current dehumanize registration and test steps, with executable examples using the English locale.

Links the guide from the documentation index. No locale data or runtime behaviour changes.

Validation on Python 3.13:

• All 18 documentation examples pass with python -m doctest docs/locale-contributor-guide.rst.
• Full test suite: 1,902 passed, 99.93% coverage.
• All repository lint hooks pass, including a separate check of the new page.
• Documentation lint and the HTML build pass with warnings treated as errors.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (2224255) to head (5714bcc).
✅ All tests successful. No failed tests found.

Additional details and impacted files

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  Coverage   100.00%   100.00%           
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  Files           10        10           
  Lines         2315      2315           
  Branches       358       358           
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  Hits          2315      2315           

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