[3pt] Replace the term вложенный модуль with подмодуль/субмодуль

[patiencedaur]

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Root document: all the docs

What's wrong?

The term submodule is currently translated as вложенный модуль, which creates a confusion as we also use the term встроенный модуль for built-in modules. Also, the current translation rather corresponds with the concept of "nested".

@ NickVolynkin, @ veod32, and I suggest replacing this translation with a more clearer one — подмодуль or субмодуль, check with @ Totktonada to see which one is better.