chore(examples): update userbook example to use avl_pager

[leohhhn]

Description

Updates the r/demo/userbook example to use the avl_pager package.

Contributors' checklist...

• Added new tests, or not needed, or not feasible
• Provided an example (e.g. screenshot) to aid review or the PR is self-explanatory
• Updated the official documentation or not needed
• No breaking changes were made, or a BREAKING CHANGE: xxx message was included in the description
• Added references to related issues and PRs
• Provided any useful hints for running manual tests

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Codecov Report

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