Add page for Storj-managed vs. user-managed encryption

[mobyvb]

Add a new documentation page as part of app/learn/concepts/encryption-key, which details the differences between "Storj-managed" and "user-managed" encryption passphrases for new projects.

https://github.com/storj/storj/issues/6980

[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 5ca08b9):

https://storj-docs--pr404-managed-encryption-q5pzgiij.web.app

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[github-actions[bot]]

Visit the preview URL for this PR (updated for commit 31a3893):

https://storj-dev--pr404-managed-encryption-ldvsm9ou.web.app

_{(expires Tue, 15 Oct 2024 14:03:36 GMT)}

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