docs: add repl-playground example for the durable REPL

[ben-reitz]

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[github-actions]

Thanks for your interest in Cloudflare Computer.

This repository does not accept unsolicited pull requests. Please use one of the accepted contribution paths instead:

• Bug reports, regressions, and fix proposals: https://github.com/cloudflare/computer/issues/new/choose
• Feature requests, enhancements, and design proposals: https://github.com/cloudflare/computer/discussions

If a maintainer asked you to open this pull request, they can add the allow-pr label and reopen it.