docs(testing): setup guide

[amara-vo]

Description

This PR adds a Beta/Pre-Release Testing Setup Guide to the documentation.

The guide is specifically tailored for non-developer users who want to test the develop branch of Booklore using Docker. It provides a safe, sandbox approach to ensure testers can provide feedback without risking their primary library data.

🎯 Key Highlights

• Target Audience: Tailored for non-developer beta-testers, but currently only promoted in the #pre-release-testing Discord channel.
• Scope: Limited to local development/testing environments only. (Need help with documentation for other environments.)
• Safety First: Includes specific instructions on renaming containers and using separate folders to prevent data conflict.

🕵️ Visibility Status

To keep the documentation clean while we refine the testing process:

• The page is currently hidden and unindexed (using unlisted: true).
• It does not appear in the sidebar or search results.
• Direct Access Link/Slug: /docs/testing/testing-setup

🛠 Changes

• Added initial docs/testing/testing-setup.md
• Created a dedicated testingSidebar in sidebars.js for related docs in future (currently inactive/hidden from main nav).