Body part names refactor

[Qrox]

Summary

SUMMARY: None

Purpose of change

Use translation for body part names, but do not merge name_plural and name into a single object. Closes #37697, closes #37700.

Describe the solution

1. Renamed bodypart name variables to reflect their actual function.
2. Added comments to explain why name_plural and name are not merged into a single object.
3. Used translation for bodypart names.
4. Updated the string extraction script and the documentation

Testing

Started game and opened character info menu and armor sorting menu, and bodypart names were displayed correctly.

Additional context

Copied from the comments:

    // This is NOT the plural of `name`; it's a name refering to the pair of
    // bodyparts which this bodypart belongs to, and thus should not be implemented
    // using "ngettext" or "translation::make_plural". Otherwise, in languages
    // without plural forms, translation of this string would indicate it
    // to be a left or right part, while it is not.