docs: Remove submodule related content

[Yaminyam]

Since we no longer use submodules, I think the submodule stuff is confusing to users.
So, I delete the content related to the submodule.

Checklist: (if applicable)

• Milestone metadata specifying the target backport version

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📚 Documentation preview 📚: https://sorna--2383.org.readthedocs.build/en/2383/

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📚 Documentation preview 📚: https://sorna-ko--2383.org.readthedocs.build/ko/2383/

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