Document how stage-packages differs from installing a package on the system

[lengau]

What needs to get done

Once staged packages are fully implemented (#1839, #1844), write documentation to explain how staging a package differs from installing it in the resulting system and how to use operator libs to achieve this instead.

Why it needs to get done

Users who use other crafts are likely to expect stage-packages to work the same way in charmcraft. However, because of how fundamentally different charms are from snaps and rocks, they won't. See #1990 for an example.

[syncronize-issues-to-jira]

Thank you for reporting us your feedback!

The internal ticket has been created: https://warthogs.atlassian.net/browse/CRAFT-3676.

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