docs: add manual connection note and approval warning for personal-files

[DRAKKKkk]

Fixes #6261

Updates the "Craft a snap" tutorial to clarify the behavior of the personal-files interface. Specifically, it adds a note explaining that personal-files does not auto-connect and requires manual store approval.

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• I've followed the contribution guidelines.
• I've signed the CLA.
• I've successfully run make lint && make test.
• I've added or updated any relevant documentation.
• In documents I changed, I added a meta description if one was missing.
• I've updated the relevant release notes.

[DRAKKKkk]

Hi @bepri, @mr-cal, and @canonical/snapcraft-authors,

This is a quick documentation update to clarify the manual connection and store approval requirements for the personal-files interface (Fixes #6261).

I look forward to your review and feedback. Thanks!